# New member needs help -> Rough idle after changing dwell angle on 389cu inch. 1966 Pontiac



## Maczuga (Sep 16, 2020)

Hello guys,

I am new here and I apologize if this topic was already solved (tough I couldn't find my exact problem in the forum).
So to let's start with, I am owner of what looks like to be quite a survivor, a 1966 Pontiac Star Chief Executive 4-Door HT with the 389cu in engine and TH400 transmission.
The car was shipped to me (Poland) from the State of Kansas and has probably original 36k miles on it (I searched back all the previous owners, who happend to be two older women and a man). The interior looks very good (tough the dash pad is multiple times cracked), car had one repaint (recent paint is still on lead basis so I assume the job was done at least in the 80s or earlier) and only two corrsion holes, one in the trunk and right quarter panel. As far I can consider (I am no mechanic) the car seems to be all stock and had even original sparkplugs and wires. So no modifications are noticeable.

Now to get to the problem of mine, the car was a runner since the first start I have made, so there are no start up issues. But the problem occures after warming up and leaving the car idling. At the very first time the idle rpm's sat at 1600 and it was very hard to lower them to ~1350 (lowest possible before stalling). This realted to a higher vacuum and resulted in heavy transmission jerks while putting into D or R gear. That's why I gave the car to a mechanic and he did the following things:

1) disassembly, cleaning and changing gaskets of the carb (two barrel Rochester) it turned out there was a vacuum leak right at the base of the carb, at firewall's side,
2) disassembly, cleaning, repainting of intake manifold + valve covers, replacing gaskets,
3) full fluid flush of engine, transmission, powerbrakes, powersteering and cooling system + chaning filters,
4) replacing spark plugs (AC 45RS put in),
5) disassembly, cleaning and reapainting of fuel tank,
6) changing of some cracked vacuum lines around carb,
7) enigne tuning.

The day I picked up the car and drove back home (~20 miles), idling was good at 650 rpms (measured whilst put in D, dashpot unhooked and sealed). I wanted to make sure that the mechanic did everything alright, so I checked my Pontiac manual and looked up for all other things to look after and stumbled upon "engine dwell". I have the "check" window in my distributor cap so I just hooked up my car multimeter and begun to measure while having the engine idling. It clearly stated 14° and had very little play when I changed the motors rpm's (like 13,9 - 14,2°). The manual says that the dwell should be placed at around 30° +/- 2 so I took my alan screw and increased the dwell to exact 30° by steps of 5°- adjusting each time timing back to 6° BTDC . Doing this maneuver I had hooked up all the time my dwell meter and disconnected distributor vaccum as well as cabin+dashpot, all sealed. After reaching the 30° I checked back timing, and after that I looked up the rpm's - and surprisingly they were way off at 1.100 with waves of ~150 up and down.
Readjusting any of the carb screws (fast idling screw or both fuel mixture screws) did nothing. Going back with the dwell also didn't work. Installing a vacuum gauge clearly showed a vaccum fluctuation between 17,5 - 14,0 ins Hg on idle. Tranny still kicks like a horse into D/R
The engine parameters are now as follows:
Timing: 6 BTDC
Dwell: 30°
I have on my own tried to look for some vacuum leaks but found none.
Changed now also all spark wires with distributor cap and rotor.
Engine starts with set choke at once, almost instantly, doesn't even take a full turn of the starter.

Do you have any suggestions what might happened to the engine?

P.s.: I did not check yet the compression of each cylinder (36k miles).
I noticed strange knocking coming off the right side of the engine, maybe below the block, only after warming up and while stronger accelerating the car while driving. The frequency of the noise is constant and does not relay to the engine's rpm. Might that be a loose exhaust manifold heat valve? This noise was present ever since I first drove the car.
I fill the tank with Shell's V-Power Racing (100 octane) + Kleen Flo Nitro Additive
Below: Picture of the vacuum modulator of the TH400, Engine Bay and my first trip with the car

Thank you for any replies and greetings from Poland 

Artur


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## Lemans guy (Oct 14, 2014)

Your vacumn fluctuations may be moving your idle dashpot,...those were designed to not allow the throttle to slam shut on fast deceleration, like coming to a fast stop....then the car would stall. So the dashpot cushions that fast shutdown.....many guys just disconnect them.

Now just for a test you can disconnect it and plug the vacumn line. They at warm idle reset the dwell to 30 degrees but leave the distributor vac hooked up. You want the dwell to be 30 degrees when the engine is idling like it normally will. Once at 30, then disconnect and plug the vacumn to the distributor and reset your timing to 6 degrees BTDC. This is just a check and test, I think your idle dashpot is moving in and out from fluctuating vacumn. Which may be from another vacumn leak, that vacumn modulator on the trans looks rough and probably leaking. Those are about $15 from Summit, Ames, JEGS ...and are easy to install. 

But you can also clamp off all other vacumn, except distributor,..which you may be doing to see if it improves your idle...

It is some give and take. Also your dashpot for the throttle may be leaking, and a weak throttle spring, that big one in the picture can make idle fluctuate...

Great looking Pontiac, super color, love the vent windows, Polish people were the greatest American allies, their bravery in every action in WW2 is legend....

This noise needs more investigation could be exhaust heat riser valve leaking.....


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## Maczuga (Sep 16, 2020)

Lemans guy said:


> Your vacumn fluctuations may be moving your idle dashpot,...those were designed to not allow the throttle to slam shut on fast deceleration, like coming to a fast stop....then the car would stall. So the dashpot cushions that fast shutdown.....many guys just disconnect them.
> 
> Now just for a test you can disconnect it and plug the vacumn line. They at warm idle reset the dwell to 30 degrees but leave the distributor vac hooked up. You want the dwell to be 30 degrees when the engine is idling like it normally will. Once at 30, then disconnect and plug the vacumn to the distributor and reset your timing to 6 degrees BTDC. This is just a check and test, I think your idle dashpot is moving in and out from fluctuating vacumn. Which may be from another vacumn leak, that vacumn modulator on the trans looks rough and probably leaking. Those are about $15 from Summit, Ames, JEGS ...and are easy to install.
> 
> ...


Thank you for your reply,

for the dashpot - it was recently changed for a NOS (I test sucked it before installing). The engine tuning I did was with the daspot and cabin vacuum lines hooked up. I even plugged the one from the power brakes. It had no impact on the idling at all. For the tranny vacuum - I did not touch them, have to test it!
However as for the carb itself, neither the idle mixture screws really work (either the engine just runs or it stalls with a weird noisy knocking orchestra . The acceleration screw is free and doesn't touch the acc. lever at all. I can also disconnect all other levers on the carb and the engine won't die. I'll try to add a short clip I made with my phone. There you can see the engine running with all the levers off on the carb. (Okay, had to post it on Youtube 



).
The most curious thing is that all the problems above just returned (formerly the mechanic fixed most of it) after I changed the dwell from 14 to 30 deg. After that I thought maybe some electric bypass formed on the ignition system due to the old wires, so I changed them along with the distributor cap and rotor, but unfortunately it changed nothing.

Ps.: The modulator was (or still is) covered in heavy mud, so was all the engine bay before I started to clean it up. The car came from Topeka (Kansas), must have had some dusty roads back then.

Meanwhile I will try to hook up the tranny vacuum and see what happens.

Thanks,

Artur


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## Lemans guy (Oct 14, 2014)

You may need a new set of points,...remove the dist cap and look at them are they pitted or burned,..in your write up I didn’t see where you changed them...but I may have missed it. Changing the dwell is basically just moving the points, so if they are original they may be bad.


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## Maczuga (Sep 16, 2020)

Nah, nothing burned or rusted etc (besides the centrifugal counterweights). For my layman eye the points look almost brand new. Below some pics (sry. for the quality, it's very difficult to get a good angle on that tight spots).
First - engine as it was when I received the car, secondly - one of the original spark plugs (the wires are code dated with Packard Radio TVRS LR4-Q 65, on cylinder 6 there was a current breakdown on the shoe, I could here the spark missing the plug), next view of the original distributor cap - untouched following distributor with points.


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## Lemans guy (Oct 14, 2014)

I would make sure those screws holding the power wire and the ground wire at the points are clean and tight connections. Also the screws holding the points down make sure they are not loose, as this happened when you adjusted them with the Allen wrench.

You need to as time and money allow change that distributor cap and rotor plugs and spark plug wires, as 50 year old spark plug wires and spark plugs do deteriorate, but I don’t see any obvious signs that would be making it hunt for idle.

Now the rubbing block on the points may be worn down considerably and when you adjusted the dwell of course you move the points and it moves the rubbing block. The points have spring tension and that fades as well, so when you moved the points it may have effected how well the points can stay hard against the cam, because of wear....

I still think I would change the points, and reset not a large expense, and a good idea anyway. A normal rune up would change them and spark plugs as well.

Stay with it you will get it, keep that plug gap at 35 ....


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## Maczuga (Sep 16, 2020)

Oh, I must have missed that part in the description. Sorry. The rotor cap, distributor cap as well as spark plugs and spark plug wires have been already changed. I look forward to change the points and in the meanwhile I’ll hook up also the tranny vacuum line.


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## Maczuga (Sep 16, 2020)

Lemans guy said:


> I would make sure those screws holding the power wire and the ground wire at the points are clean and tight connections. Also the screws holding the points down make sure they are not loose, as this happened when you adjusted them with the Allen wrench.
> 
> You need to as time and money allow change that distributor cap and rotor plugs and spark plug wires, as 50 year old spark plug wires and spark plugs do deteriorate, but I don’t see any obvious signs that would be making it hunt for idle.
> 
> ...


Speaking about the distributor, I noticed some horizontal play (not excessive, just a few mm) of back and forth of the distributor cap while being mounted (screwed tight) on the distributor shaft socket. Is this normal?


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## Old Man Taylor (May 9, 2011)

One step at a time. I fear your dwell meter reading is not correct. If the engine ran well at 14 degrees, then there is something wrong with your dwell meter reading. I wonder if the setting is on 4 cylinder. If you double 14 you get 28, and that is within the spec range for the dwell - 28-32. I would put it back to the 14 degree setting (measured exactly as before) and go from there.


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## Maczuga (Sep 16, 2020)

That’s a tought, gonna check that. Also, in theory, if the points were messed up, wouldn’t this result in a fluctuating dwell angle reading (like more than 1-2 degrees)?


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## Old Man Taylor (May 9, 2011)

Maybe, but normally the dwell just gets bigger until the points are really bad.


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## Mine'sa66 (Oct 30, 2019)

Old Man Taylor said:


> One step at a time. I fear your dwell meter reading is not correct. If the engine ran well at 14 degrees, then there is something wrong with your dwell meter reading. I wonder if the setting is on 4 cylinder. If you double 14 you get 28, and that is within the spec range for the dwell - 28-32. I would put it back to the 14 degree setting (measured exactly as before) and go from there.


That was my initial thought.


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## Lemans guy (Oct 14, 2014)

Your rotor cap should be tight and not have side to side play, look under the side of the distributor will a little mirror once you put the cap on and make sure it is sitting inside the groves where it is supposed to be. I had a distributor recently that was on a Pontiac and someone had put it on not even near the two slots underneath. They are easy to miss, when on snug and right it has no play.


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## Maczuga (Sep 16, 2020)

Mine'sa66 said:


> That was my initial thought.


I’ll get that tomorrow checked


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## Maczuga (Sep 16, 2020)

Alright guys,

that's what I have noticed so far:

1) clamping off the transmission vacuum line and sealing off the carb's socket resulted in:
rpm's (x 10) before clamping off: 155 - 179 (amplitude of 24)
rpm's (x10) after clamping off: 131 - 163 (amplitude of 32)
- both parameters measured on hot idle,
That is about 12% difference on average idle speed between hooked/unhooked tranny line,
2) measured again the dwell, with 8-cyl setup on my multimeter and it gave me 30,1 - 30,2° 
3) checked the screws distributor point's - they were already pretty tight, however there is a play between the distributor shaft and the rubber pad of the points (see link below).






P.s.: I wasn't absolutely correct while I wrote about the cap being loose, it sits absolutely tight on the centrifugal socket, but the shaft as mentioned above has some play.


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## Lemans guy (Oct 14, 2014)

The distributor moves like that when the weights deploy outward, that is ok.

I would change the trans Vacumn modulator and use a new rubber line, you have a vacuum leak there that is increasing your Idle RPM..

with Dwell at .30 you are good...did you have the meter on 4 cyl as Mr. Taylor and Minesa66 noted


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## Maczuga (Sep 16, 2020)

Lemans guy said:


> The distributor moves like that when the weights deploy outward, that is ok.
> 
> I would change the trans Vacumn modulator and use a new rubber line, you have a vacuum leak there that is increasing your Idle RPM..
> 
> with Dwell at .30 you are good...did you have the meter on 4 cyl as Mr. Taylor and Minesa66 noted


You mean exchaning the factory steel tranny vacuum line with a rubber one? Also, there are many modulators on the web to buy, are there any particular I need to look for(e.g. any one that would fit the factory look)?

Thanks for all the help


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## Lemans guy (Oct 14, 2014)

No you can use the steel line, use what you have...AMES should be able to get you one that matches or Original Parts Group Inc (OPGI)...

if the line is steel and not leaking keep it, some have rubber going from the steel line to the modulator and they deteriorate...

You want a new modulator with no leaks


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## Maczuga (Sep 16, 2020)

Lemans guy said:


> No you can use the steel line, use what you have...AMES should be able to get you one that matches or Original Parts Group Inc (OPGI)...
> 
> if the line is steel and not leaking keep it, some have rubber going from the steel line to the modulator and they deteriorate...
> 
> You want a new modulator with no leaks


I crawled today under my fierce charriot and cleaned up that modulator. I made a suck test (it’s air tight) and replaced the little rubber connector with a new one, altough I am going to replace the modulator asap. I’ve got two questions about this area, one- what good for is the little screw atop the modulator (do I adjust with it the shifting steps)?
Secondly, the two steel lines going right into the tranny’s casing, are those the cooling lines?
Also, a question to determine the car’s history, I roughly cleaned a small part of the bottom panel on passengers sight from all the dirt and out came this quite good looking surface finish- what are the odds this could be the factory underlay protective paint? If not, what finish should I use to match factory style?
















Thanks for replies,
Artur


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## Mine'sa66 (Oct 30, 2019)

Maczuga said:


> I crawled today under my fierce charriot and cleaned up that modulator. I made a suck test (it’s air tight) and replaced the little rubber connector with a new one, altough I am going to replace the modulator asap. I’ve got two questions about this area, one- what good for is the little screw atop the modulator (do I adjust with it the shifting steps)?
> Secondly, the two steel lines going right into the tranny’s casing, are those the cooling lines?
> Also, a question to determine the car’s history, I roughly cleaned a small part of the bottom panel on passengers sight from all the dirt and out came this quite good looking surface finish- what are the odds this could be the factory underlay protective paint? If not, what finish should I use to match factory style?
> View attachment 137325
> ...


The 2 steel lines are for the transmission cooler. They run to the radiator, or if so equipped, an auxiliary cooler. The screw on the modulator is for adjustment. Most modulators are sealed and not adjustable. The vacuum modulator delays the auto shifting of the trans. You can vary the shift point via the screw. If it's shifting well, I would not mess with it. As far as the originality of the floor paint, I will definitely defer to the Pontiac/body man experts.


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## Maczuga (Sep 16, 2020)

Mine'sa66 said:


> The 2 steel lines are for the transmission cooler. They run to the radiator, or if so equipped, an auxiliary cooler. The screw on the modulator is for adjustment. Most modulators are sealed and not adjustable. The vacuum modulator delays the auto shifting of the trans. You can vary the shift point via the screw. If it's shifting well, I would not mess with it. As far as the originality of the floor paint, I will definitely defer to the Pontiac/body man experts.


And I think I might be missing here sth? This is a picture made from the underside of the factory a/c box. You can see a socket with, what seems to be a broken off hose, maybe part of the condenser drain?


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## Mine'sa66 (Oct 30, 2019)

Maczuga said:


> And I think I might be missing here sth? This is a picture made from the underside of the factory a/c box. You can see a socket with, what seems to be a broken off hose, maybe part of the condenser drain?
> View attachment 137327


I _think_ that's the remains of a short piece of rubber/plastic tube for the drain.


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## rockdoc (Mar 16, 2009)

I had some problems similar to yours. I found that my vacuum advance was no good (you can test it easily by putting vacuum onto the hose leading to it, it should hold vacuum and pull in). It's an easy replacement. I also had an idle problem and, after rebuilding carb (also a 2 BBL Rochester like yours, by the way it's an easy job, you should try it next time), I found that it was the condenser. What an easy fix! And cheap, but get a good quality condenser. Be careful that all the connections are good inside the distributor.

I agree about the transmission modulator...if it's shifting OK, don't mess with it. Are you sure it's leaking?


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## BLK69JUDGE (Jun 10, 2010)

change to quality "blue streak" condenser and points they are 2 seperate parts

lots of issues for me at least with the quality of the 1 piece condenser points.... set up

or ask Mr Taylor what the delco points and condensers he uses ,,,

og might have the part number also ,,, I am not home till tomorow ,,,]

1007 ??

Scott T


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## Maczuga (Sep 16, 2020)

Hey guys, 
I will try to make a full vacuum leak test, from the engine, down to the tranny. This time maybe with propane gas, saw some recommendations on Youtube for that? Maybe I could focus after it on the distributor electrical parts if the problem should still exist. I am also thinking if the carb could be still an issue, maybe the idle passages below the jets are somehow stucked (for remainder, the fuel tank has been cleaned out and some debree might have remained and now it’s stuck in the carb channels, altougth there are three filters on the way?). When I last opened the top of the carb to check the adjustments inside the bowl, floater etc. I noticed some „sand” like particles accumulating on the bottom of the fuel chamber. Is that normal? I replaced before the fuel inlet filter and the bigger one outside the carb with the additional condenser line. 
For the distributor, I checked that „blue streak” condenser and points, as far I can tell they are made in the Netherlands, so it shouldn’t take long to get them. Having anything shipped from the US right now is a pain in the butt. Waiting for some things from Ames/Opgi now since 3 months... 

Again, thanks for replying and have a good time with your presidential election debates


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## Maczuga (Sep 16, 2020)

Hey guys,

I finally checked my whole engine bay for vacuum leaks (did the propane test) and found not a single leak . The tranny's vacuum modulator was also tested. Furthermore, I unhooked everything from the carburetor (vac lines from distributor seperate) and no notable change in engine's rpm's was given. Reminding the fact, that changing the dwell was in the time line directly related to the sudden idle rpm increase makes me question if the higher octane fuel available in Europe (we got 95, 98 and 100) requires maybe lower dwell angle?

The OEM manual states that the engine dwell should be at 28-30° for 1966 in the US (guess again that this range was for fine adjustments, depending on what fuel and altitude you were travelling). From what I've heard the octane rating for gasoline was back then way lower then 85?
When the car was first filled up by me, I poured somewhat 20 liters of 95 octane, after flushing the whole tank I assume again 95 or 98 was filled in. Because of the knocking noise I thought maybe I should pour even higher octane (Shell got 100) and added some "lead" additive with anti knocking. At this point the engine dwell was still at 14°.
By changing it now to 30° the combustion is out of order and that's why I have this high idle rpm? 
I couldn't find any information on the internet about the relation of gasoline octane rating and dwell angle.

In addition, if the rpm's are above ~850 + the idling passages in the carb are physically bypassed due to the change of vacuum flow inside the air horn and cluster, hence how am I supposed to lower the rpm's with the mixture screws?

*Just for notice, the car did around 315 miles on new gasoline since I bought it, tough it wasn't raced yet on the highway. Before that the car stood a unknown time at the previous owner.

Again, thanks for helping me out! 

Greetings from Poland.


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## Lemans guy (Oct 14, 2014)

There is not going to be a relationship between dwell and octane in the gas. Octane was a bit higher in the 60’s but not as much as everyone thinks ans they way they measured it was different.

An 850 idle is ok if your dwell is set at 30 and timing is correct, as your cam may not be original? Or the heads or piston stroke has changed. Lot’s of modified engines need a little more idle speed than the factory setting because of those changes.

Dwell is simply the the time in degrees of the distributor cam angle that the points are closed...when the points are closed the coil builds a charge, and it collapses once the points open and provides that 40,000 volts needed to fire the plugs. Keep it at 30 degrees as reducing it will give you a weak spark and poor running.

Dqell effects timing, and you may have way too much idle timing. If your vac can is pulling full manifold vac and more than 10 degrees then it is too much, your idle timing with a medium cam should be 20 to 26 in there,...like 12 from base and 10 degrees from vacumn can.....if you are pulling 12 from base and sixteen or 20 from vacumn can it will run up your idle.

When you changed dwell you changed your timing, so you need to work on the timing.


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## BearGFR (Aug 25, 2008)

Dwell Angle: It's the measurement of the length of time, in degrees of distributor rotation (not crankshaft rotation) that the points are closed. The primary windings in the ignition coil can only be 'charging'/saturating during the time the points are closed, therefore dwell angle directly relates to how much time the coil can be 'charging'. The longer it can saturate, the more energy is released (spark) whenever the points open and the magnetic field created in the primary windings "collapses" and induces a voltage in the secondary windings. From this we should be able to infer that as RPM goes up, this 'charge time' decreases, which is the reason that points/coil ignition systems produce a less powerful spark with increasing RPM.

If your "dwell angle" is too small (short), your spark will be "too weak" to light the fire especially at higher RPM. If the dwell angle is too large (long) then as long as they open at all you'll get a stronger spark, but you'll also get increased arcing when they do open which will significantly shorten their life.

"Back in the day" this was the reason that some performance distributors had two sets of points. Their timing was slightly offset from each other such that as long as at least one of them was closed, the coil was charging. The spark didn't happen until they were both open. This arrangement allowed for a longer dwell time (stronger spark) without burning up the points due to excessive arcing.

However, dwell angle even varying from one extreme to the other isn't going to be the cause of your problems. You either have enough spark energy to "light the fire" or you don't, and if you're engine is running - you're ok in that department. Just make sure you're not on the extreme end of the range that causes shortened point life.

Bear


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## Maczuga (Sep 16, 2020)

I see that, 
so maybe I'll drop that one angle to 29° of dwell. Do you have any opinion about what octane rating I should use?
In relation to the cam and or other parts of the engine, the car has now approx. 36 k miles on the odometer (on the orig. title from the -90's the car had 35600 miles written). Both owners were woman from province villages around Kansas City - that and despite the good all around good shape of the car makes me believe that noone has tinkled with it, or furthermore pulled the engine or so. 
I may be ofcourse mistaken but I assume it is all stock.
Manual says that while having the tranny in Drive and holding the brakes, the car should cycle at 600 rpm/s with the TH400, or 1030 rpm/s in Neutral with the adjusted dashpot at the throttle linkage.
If I pull the vac line of the distributor and plug the carb on warm engine (and the idle is at ~1.000 - can't lower it since it will cause jerking) the strobe shoots at exact 6+ BTDC, the idle drops the same time to around 850. Replugging the distributor the idle goes back to 1.000 and the timing rises above the 6+. I do not know what timing I should have with that 1.000 rpm/s or despite the 850, - maybe +7 or 8 BTDC?
I also checked the vacuum with a gauge (hooked it to the port where the vac for the dashpot and cabin go) and reading is almost steady (needle fluctuates only minor at 2-3 points) and shows around 18+ in of mercury (with the 1.000 rpm's!).


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## BearGFR (Aug 25, 2008)

Initial timing is >almost< of no significance. What really matters is total mechanical timing and the RPM where it occurs. What I consider "later model" (68 and later) Pontiacs with open chamber cylinder heads tend to "like" something in the general neighborhood of 35 degrees total, all in by about 2500 RPM or so. Earlier closed chamber heads can tend to like a little more than that. Both cases assume that the engine doesn't get into detonation (knocking/"rattling") under heavy load at low RPM. If you're running more or less stock cast iron heads on an otherwise un-modified engine, that can at times be a challenge and won't allow you to run optimum timing, due to the lower octane and lack of lead in today's pump gas.

That topic has been discussed on here multiple times, so you might avail yourself of the thread search tools to see if you can find those posts.

Timing is _always_ adjusted with the vacuum can disconnected, hose plugged. Always. The purpose of the can is to add "extra" timing under light load, part throttle operations because it helps both fuel economy and engine cooling, but it has nothing at all to do with max power. The only reason to even look at timing with the can "active" is if you're curious to know how much it's capable of adding, but you never adjust timing with it connected. 

Unless you know how much total >mechanical< advance you're getting, you're pretty much just spit-balling at things, trying to hit a bullseye target in a dark room. You're dealing with too many unknowns to have much success. I recommend using a light that's capable of measuring total advance. There are several on the market and some aren't all that expensive. The initial timing setting doesn't really tell you anything unless you know for certain how much "travel" there is in your distributor and the RPM where it's maxed out.

If you're getting "jerking" at less than 1000 rpm idle with the can disconnected and the hose plugged, and your timing is right, then your idle is probably too lean or you have an as of yet undiscovered vacuum leak (which causes a lean condition).

Try this: 
Once you know for certain where your timing is at:
Completely close both idle mixture screws, being careful to count exactly the number of turns so that you can open them back to where they were.
Write down where they were, then set them back there.
Disconnect the can, plug the hose (or cap off the fitting at the carb), hook up your vacuum gauge.
Start it up, start backing down the idle RPM until it starts to get rough.
Note the RPM and the vacuum reading where that happens.
Try backing out/opening up the mixture screws and see if you can find a setting where the idle "cleans up" (use your vacuum gauge, tune for the highest reading you can get)
It's a dance. Dropping rpm decreases the vacuum signal which alters the air/fuel flow through the carb and can also lower timing (if the previous rpm was already starting to activate the mechanical advance mechanism) - so when you change something you also have to check and verify the other things too. At 1000rpm+ you may already be "getting into" the carb's transition slots due to the higher vacuum and higher air flow velocity. Dropping rpm decreases the signal and the flow, and can require you to add fuel "back in" via the idle screws.
Lather, rinse, repeat until you can get it down to a decent idle at around 750 RPM. -
If you're not able to achieve a decent 750 rpm idle, then either something is wrong (you've got a vacuum leak somewhere or some other problem), or that engine has a relatively nasty cam.

It's not rocket surgery 

Bear


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## Maczuga (Sep 16, 2020)

Well, 
I will take a deep look into this the next days and altough it is no rocket surgery there's litterally no mechanic in Cracow (one of the biggest cities in Poland) who would just jump right into those things. To be honest many times I have to explain a lot and the further I get the bigger the eyes of the mechanics become  The last known car carb expert passed away several years ago and now we got no one in the whole town 
One thing makes me really wonder about the carb because if I back up the acceleration screw on the linkage all the way back (the one that lays against the cam with the choke steps) so that the inner two throttle valves (I've got the two bore Rochester) are completly shut - the engine doesn't die. I always thought that there has to be a minumum pressure from the acc. screw (to slightly open throttle valves) to keep the engine alive or is this screw only for choke adjustment? Hell, I can even disconnect the whole acceleration rod/linkage coming from the cabin and nothing happens 

For the rattling as you mentioned, it occures only under load while accelerating on a warm engine, but, the intensity does not change with higher rpm's, it is just constant. I checked the heater valve on the right hand exhaust and it is assembled, nothing loose or broken, tough quite heavy rusted?!


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## Maczuga (Sep 16, 2020)

Today I tried to redo all the adjustments without having the distributor module connected and this is what I came to:





Timing at 6°BTDC, distributor vacuum clamped off, highest possible and "steady" vacuum between 13-16 in of mercury, idling between 70 - 97 x10 , full open choke throttle, fully closed throttle of both barrels (idling screw without contact at the choke plate). After reconnecting the distributor vacuum rpm's went instantly to 129 - 145 x10


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## BearGFR (Aug 25, 2008)

A couple observations:
13-16 inches of vacuum seems to me a little bit low for a stock cam. I'd expect it to be closer to 20 or 22. As a point of reference, in the previous incarnation of the 461 in my GTO that had a solid lifter roller cam with 236/242 degrees of duration at 0.050 tappet lift still made 12"-14" of vacuum at idle, and it was definitely hotter than stock. That and what you've previously said about idle problems still makes me suspect that you've got a leak somewhere. Have you tried disconnecting the power brake booster? Examined that hose and all the other hoses and fittings for cracks? Watching your video, the vacuum source you're using for the distributor diaphragm is non-ported. That's not necessarily a problem, but it will start to add in some advance even at idle and that's going to tend to raise idle rpm some. Are you able to adjust idle down with it connected? With everything connected, what idle rpm can you achieve with the parking brake set and the car in drive? (Make sure the brake does hold the car - use some wheel chocks for additional safety - and don't be blipping the throttle!) Ask a (trustworthy) friend sit behind the wheel and hold it with the brakes. 
Maybe disconnect and cap/plug _every_ vacuum line you can find and see if that makes any difference.

Bear


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## Maczuga (Sep 16, 2020)

Will try to measure the exact vacuum readings after disconnecting every line and plugging the ports on seperate. What else could I check for leaks besides spraying around the carb stove and intake manifold + sparkplugs?!


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## BearGFR (Aug 25, 2008)

If the vacuum diaphragm in the power brake booster is leaking, you probably won't be able to directly check it. What you should see though if that's a problem would be an improvement with it disconnected and the fitting capped off. I think I remember seeing that you'd already checked around on the intake manifold and carb? Sometimes it's easier to just disconnect and cap "everything you can" and see if it gets better.


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## Maczuga (Sep 16, 2020)

Well if I plug everything, that is a) disconnect the trannys steel line and plug the port, b) power brake booster, c) dashpot and cabin line (they are on the same line), d) distributor can; I should eliminate any possible leaks from those devices.
If I see a significant change in the idle and or vac pressure after clamping off those external devices I should be able to track them sperately down by reconnecting them one by one.
Having this done I can only check the carb itself, intake manifold, pcv valve (both, the engine and manfiold port) and sparkplugs for any further vac leaks and do this with eg. carb cleaner or propane.

Do I see this right?

Just to be sure I understand this:
the „third” screw, located on the back of the carb is the idle speed screw, that regulates the choke what allows cold start and after warming up the screw should have full clearance from the choke cam (this half moon shaped plate with steps at which the screw lays again) as shown on my vid?
The carb regulation itself is only made by those two front screws aka „idle mixture screws”, one for each barrel?
Starting position would be 1,5 turns back out from the closed position, from where I would back out furthermore both screws, rotary, with possible same amount of turns until I achieve the max. vacuum on my gauge reading. Then making a half turn back inside to make sure I am not running rich?


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## BearGFR (Aug 25, 2008)

I excerpted all the pages from my '69 shop manual that address the setup for the Rochester 2GV 2bbl, which looks like what you have. I'll attach those pages to this reply.
On the idle mixture though, I'd recommend setting for best vacuum and leaving them there - don't turn them back in any. See if that changes things for you.

Bear


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## Maczuga (Sep 16, 2020)

Thank you for the pages, from what I know I have a Rochester 2 Jet 2G carburetor. I have adjusted some time ago all the linkage and float level according to my factory manual (the pages are identical to yours.
Unfortunately none of this operations helped, I always end with the same setting: Carb’s idle speed screw can be backed out fully and even if I disconnect the acc. rod coming from the cabin, thus relieving the throttle lever fully the engine runs „by himself” on ~1100 rpm’s and the mixture screws doesn’t really change much (probably because rpm’s too high). Before the carb was rebuild, there was the same issue with the rpm’s but back then faulty was a gasket under the carb with a big vacuum leak on the backside (spraying carb cleaner on that spot Immediately killed the engine). Now something much more dodgy must have occured. Can’t get a clue on that one ,... and it all happened after I raised that dwell from 14 to 30 degrees...
What eventually comes to my mind now is that when I put on the aircleaner, thus lowering the vacuum noise I can hear a clear clicking noise like every half second, might be electrical, it is very gentle. Was thinking first that maybe some short cut found it’s way on one of the sparkplugs but I definitely changed all wires and rechecked tightness of all connections. Might the distributor just give this noise under normal working condtions?

Ps. I did not change yet the points and condenser since the dwell reading is very stable (+/- 0.1 degrees, so it should be pretty healthy?) and besides that shipping from US is now with Covid very cumbersome.


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## PontiacJim (Dec 29, 2012)

You have a problem if you were to disconnect the carb from the pedal linkage and close the throttle butterflies and the engine doesn't stall out. Try that.

Have you tried that? Is it 1.) possible that you gas pedal linkage is holding the carb throttle open? 2.) that your throttle valves in the carb were not centered in their bores when the carb was rebuilt and now will not close completely and thus shut all incoming air/fuel and allows the engine to continue to run? You can check by pulling the carb off, closing the throttle valves and shining a flashlight up through the bottom as you look down inside. You should see no light.

That on/off clicking you hear, if at the back/firewall, could simply be the voltage regulator points opening and closing - or possibly the Alt.

Ignore the highest vacuum reading for now. Can you adjust the idle mixture screws to get a smooth idle? Try that. I typically seat the idle mixture screws, then back out 2 1/2 turns and go from there. Back them out first, one side at a time, and see if it will run better. Then run the screw in until the engine begins to run rough, then stop. Back it out again until engine runs smooth and stop - idle mixture set. The do the other side.

Your idle will jump up, as well as your initial timing when you reconnect your vacuum line to the distributor. Once it increases, then you should be able to adjust your idle speed found on the side of the carb on the 'low" notch of the fast idle cam and choke wide open.

Don't know how old your engine is, but it is possible the timing chain/gears are worn and sloppy, or even the harmonic balancer outer inertia ring has slipped and is inaccurate. 6 degrees BTDC could be completely wrong. With vacuum disconnected, adjust the distributor by ear and see if the engine smooths out. So going by factory specs on a worn/old engine may not work.


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## BearGFR (Aug 25, 2008)

Wait... you're saying that if you back the idle screw all the way out it STILL runs? That should not happen. My first suspicion is like Jim's, that whoever had the carb apart last may have removed the throttle plates to clean everything and then didn't get the throttle plates reinstalled correctly. There are two things to check there. One is relatively easy: remove the carb and see if they're completely closing. You can use a light as he suggested to verify. The other thing that might be wrong is they may have been reinstalled upside down/backwards. The leading and trailing edges of the plates have a slight bevel on them so that they will seal flush against the throttle bores when they shut. If those edges are 'backwards' then they won't seal very well and can leak air. Getting the plates centered in the bores is relatively easy: just loosen the screws so that they can move, hold the throttle shut - that will force them to align themselves in the throttle bore, then tighten the screws down while holding the throttle shut. To check the bevel, open the throttle all the way so that the edges of the plates are visible. You should be able to see the bevel(s) on the edge(s) of the plates. They should be situated so that when shut, the whole edge of the plate is "flat" against the throttle bore. If they're not oriented correctly, remove the screws, and "turn them over" so that they are, then re-center in the bores as previously described. 

The engine should not be able to run with the idle speed screw backed all the way out.

Bear


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## Maczuga (Sep 16, 2020)

Dissasembled today the carb and checked the throttle plates for tightness and fit. Seem to be alright, test light passed altough at a specific light angle a sun eclipse shine is present, however it‘s not possible to change much since the whole base is secured via pressfit rod and you can‘t take it further apart. Only centering would be possible but they are pretty centric. In addition I might say that the plates are slightly smaller in diameter then the bore. The edges of the plates rest tight against the bore walls. After reassembling clamped all vacuums off and watched my gauge. It rose only 1-2 points so I guess there are no leaks. Also sprayed a lot of carb cleaner all over the place without result.
Carb still running by itself at ~1300 rpm’s with 17/18 in hg. Idle screw backed all way out 👍🏻
Playing with the timing also did not help  Going off that 6 BTDC made it only worse, stuttering,vacuum drop.
Might the float level inside the carb have any impact or maybe a leaking powervalve?


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## PontiacJim (Dec 29, 2012)

I am seeing daylight in pic #1 on the lower of the 2 throttle blades. I can also see slight daylight in pic #2 on the right edge of the throttle blade. But I don't think enough to be of concern.

The shaft is not pressed in. This screws would need to come out of the throttle plates (and the screws are typically peened so they don't back out and need to be ground on first so you can remove the throttle plate screws or they will break being brass).

The throttle blade shaft in pic #1 at the top looks dark near its end against the casting. Is it wet? If the throttle shaft is worn, it can suck in air and will act as a vacuum leak and raise RPM's. Spraying carb cleaner etc. may not show this up. I don't know if they have a kit to install new bushing and shaft as they do for QuadraJets which often wear out due to age/use.

You engine should not have run poorly as you stated if you advanced the timing, it should have increased in RPM - the same as when you connect the vacuum advance. If you retarded it, it may have run poorly and it can also cause the engine to run hotter. So one direction should have cause the engine RPM to go up and should have run smoother, then other would have decreased the RPM and probably ran poorer.

So when you closed the carb completely, throttle cable disconnected, your engine still idled at 1,300 RPM"s? I can't see how with the throttle blades completely closed.

I looked at the first picture of your engine. I see the dashpot (the round canister) on the driver's side with the vacuum hose which appears to be extended and I don't think your engine is running. The rod has a nut on the end of it to adjust this. It looks like it is up against the throttle arm which would keep the throttle blades open and keep idle speed up. Make sure that is adjusted correctly. Try screwing it in to shorten it and see if that changes your idle speed. 

The choke housing uses a tube from the intake to heat the air for the bi-metal spring. Is it possible that the tube going into the manifold has rusted out/through and is the source of a vacuum leak? Have you checked this for any vacuum leaks? I cannot recall if there is an opening/port that connects the choke housing directly into the carb body, but I believe some do, just not sure what years.

Take a look at this drawing and read the text. If the idle air bleeds are plugged up, they can cause problems as if the engine was running rich and the RPM's won't drop. We had a member with a tri-power have this issue and tried quite a few things only to find out that the idle air bleeds for his application were too small with his big cam. Once he drilled them larger, his idle came down to a normal setting and that cured his fast idle issue.






Rochester 2 Jet Idle Circuit Technical







www.carburetor-parts.com





You verified again your float level? Did you make sure your power valve ( that long rod with the spring on it) inside the carb body was working up and down and not stuck?

Have you tried pulling a spark plug or two to see what they look like? If by chance the carb does have an internal leak, you may find a spark plug that is wet or very black from running rich.

The 2 bbl is fairly simple and really an easy rebuild. Just seems odd you are having this problem. Make sure you are using the correct base gasket, which looks like you are. It should be rectangular and only have the 2 holes for the carb barrels. Some have contours depending on intake, but I believe your should be essentially rectangular with just the 2 holes.


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## Maczuga (Sep 16, 2020)

Thank you Jim and all other for those comprehensive ideas but I'm afraid I can't handle too much of this information since I am a greenhorn in cars mechanic, despite the fact that I am dealing the very first time with an american classic. Maybe the problem lies somewhere we did not think of, even more trivial then the ones you've stated.
Let me summerize:
1) I bought this car with an already high idle (was about 1.400 rpm if I can still remember),
2) The car had never starting problems, ignition was almost instant,
3) I gave the car to a mechanic (later turned out he wasn't quite trustworthy) because the car had to have replaced all fluids, some minor body work, rebuilding brakes, engine overheated and what was most significant, it just slammed into R and D gear (probably because of those high rpm's),
(the engine was overheating when at traffic jams, but after changing a) spark plugs for new, b) flushing myself all the radiator (there was a lot of butter) the "idiot" light showed up never again.)
4) The carb was dissassembled (it looked just gross, overgrown with thick layers of oil and mud), so was the intake manifold and top cover of engine (the one between the valve cover, under the intake manifold), all gaskets at that point has been replaced),
5) The day I was going to pick up my car I ran instantly into a problem, for one there was really strange knocking coming from the engine bay (turned out it was the loose distributor cap with the rotor bangnig against it...) and because the mechanic did sth. wrong with the carb adjustment, my exhaust muffler exploaded... I did leave the car to have this repaired at once, and picked it up on the next day. Arriving at home (driving somewhat 25 miles without issues) I ran some diagnostics by myself because I did not want to put any more trust into this sloppy work of my mechanic. So I measured the rpm's on warm engine (they were quite fine, steady at around 750, with the gear in D I could even go down to 600 by backing out slightly the idle screw), checked timing (was at 6° BTDC), checked the vacuum, reading was at around 19/20 and last but not least I looked for the dwell angle and here it happend 
The meter read 14°! I took my alan wrench, opened the little service window at the distributor and rose it to 28,5°. The engine responded to this immediately with higher rpm and greater timing. Cant remember if the vacuum can was disconneted at that time (like I said, I am learning by doing  but I surely measured at some point the BTDC with the distributor vacuum hooked up.
This was the point where I have lost completly control over the engine's rpm. Could that be just a coincidence? First I came up with the idea of setting the dwell back to 14° but that did not work anymore. Next I checked all the wiring, in fact some of the spark plugs were loose and I had to tighten them (courtesy of my mechanic) and on cylinder 5 there was a shortcut (everytime I squeezed my hand between the A/C compressor and engine block to reach the spark plug I got shocked . I looked up my distributor, the cap was in my opinion in quite a good shape (the pic is at the beginning of this topic) like it should be for 36k miles, however the rotor showed a little bit of burn at the very end of the metal plate (it was also changed for a new one). Wires were original (dated with 4-Q-65 PACKARD RADIO TVRS LR), they were not brittle, anyways I changed them for new silicon ones from AC DELCO Professional - and it did not have any impact on the engine rpm or anything else. Sparkplugs were, possibly, also factory, AC 45S (picture below, the first two are as they came out from the engine, third one I just cleaned for commparison). The mechanic put in before new ones - AC 45 RS (dunno if that's bad or good). Old orignial are a) points mechanics, b) condenser, c) coil, d) wire coil-distributor, e) vacuum can.
I even readjusted all the levers and float to factory settings - nothing helped.

Maybe you can come up with some new thoughts 

Meanwhile I will do as you stated Jim, check again the power valve etc.

Re: Your drawing: As stated:"

Set the idle speed to manufactures specifications. When the RPM is too high, the idle circuit is bypassed and adjustments will not affect anything."
The rpm is in fact way too high, and I am afraid my idle circuits are just bypassed all the time. I still believe that the engine is stock so there would be no use of drilling bigger air bleeds? In fact, I rechecked and recleaned yesterday all the carb with exemption of this curious T-shaped metal piece with a spring under it, and possibly the checkball? Did not touch that since it seems to be very fragile and I don‘t really know what it does. . However all other passages in the carb are super clean. If the power valve is leaking I can not say, I will check that again but I am not sure what else I could do besides just looking at it and trying to pressfit into the guide slide.

P.s.: Trying to answer your previous questions:

As I said, the plates are centric, however they are slightly smaller in diameter then the bore (maybe because of heat extension?) and there is coming at specific angle a small amount of light through, besides that the plates are not closing perpendicular to the bore but what I think at a 10°angle as engraved on the top of em,
Excuse my misinterpretation, not the shaft but the end of it, at which sits the outer lever is fixed together along with the shaft by means of a pressed nut or rivet to the casting, don't think I could disengage this without damage.
The shaft on the pic is wet because of carb cleaner remnants  However I do recall of gasoline leaks when I was pumping "via the acc. rod" gasoline into the carb when I was checking the thigtness of all the linkage. The amount of gasoline was sufficient to wetten the gasket under the carb - this leak was strange to me.
Yes, advancing the timing raised rpms but it didn't smooth it out, it was just worse, almost stuttering, either way if I advanced or backed the timing from those +6°. There was no way of smooting out with the idle mixture screws.
Disengaging all the linkage, backing out fully idle screw let the carb in a ghostly manner still running at ~1300 rpms (there are highs and drops of approx. 100) ,
The dashpot is a nos, the original one was stucked, after breaking it free it was leaking. However it is disconnected since it's working conditions are not given (as the manual says, the dashpot should raise the idle rpm to 1030 in order to be fully working),
As I reassembled yesterday the carb, I also had to reconnect the choke warm up tube, going from the housing inside the intake manifold. As far I have even extra lubed with some heavy duty grease the threads of the upper mount/port I can't speak for the tight sit inside the manifold. All I did was spraying lots of carb cleaner onto it and it was tight,
I will look after the sparkplugs again.









Again thank's for your advice and help!

Artur


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## BearGFR (Aug 25, 2008)

Maczuga said:


> 1) I bought this car with an already high idle (was about 1.400 rpm if I can still remember),
> 2) The car had never starting problems, ignition was almost instant,
> 3) I gave the car to a mechanic (later turned out he wasn't quite trustworthy) because the car had to have replaced all fluids, some minor body work, rebuilding brakes, engine overheated and what was most significant, it just slammed into R and D gear (probably because of those high rpm's),
> (the engine was overheating when at traffic jams, but after changing a) spark plugs for new, b) flushing myself all the radiator (there was a lot of butter) the "idiot" light showed up never again.)


There's a good chance that the overheating was at least partially related to the high idle, because even at a stop it's putting the engine under load. It's probably tending to heat up your transmission fluid as well. Of course having "butter" in your cooling system wasn't helping either.



> 4) The carb was dissassembled (it looked just gross, overgrown with thick layers of oil and mud), so was the intake manifold and top cover of engine (the one between the valve cover, under the intake manifold), all gaskets at that point has been replaced),
> 5) The day I was going to pick up my car I ran instantly into a problem, for one there was really strange knocking coming from the engine bay (turned out it was the loose distributor cap with the rotor bangnig against it...) and because the mechanic did sth. wrong with the carb adjustment, my exhaust muffler exploaded... I did leave the car to have this repaired at once, and picked it up on the next day. Arriving at home (driving somewhat 25 miles without issues) I ran some diagnostics by myself because I did not want to put any more trust into this sloppy work of my mechanic. So I measured the rpm's on warm engine (they were quite fine, steady at around 750, with the gear in D I could even go down to 600 by backing out slightly the idle screw), checked timing (was at 6° BTDC), checked the vacuum, reading was at around 19/20 and last but not least I looked for the dwell angle and here it happend
> The meter read 14°! I took my alan wrench, opened the little service window at the distributor and rose it to 28,5°. The engine responded to this immediately with higher rpm and greater timing. Cant remember if the vacuum can was disconneted at that time (like I said, I am learning by doing  but I surely measured at some point the BTDC with the distributor vacuum hooked up.


Your muffler probably exploded because the car was running very rich and was passing a significant amount of unburned fuel into the exhaust where it then accumulated in the muffler. When it got hot enough and had accumulated enough fuel vapor - BOOM!

With a factory points ignition system, changing the dwell angle will also change ignition timing. This happens because when you change the dwell, it also changes the "point" on the distributor cam where the points open, and that's what causes the spark energy to be release from the coil.



> This was the point where I have lost completly control over the engine's rpm. Could that be just a coincidence? First I came up with the idea of setting the dwell back to 14° but that did not work anymore. Next I checked all the wiring, in fact some of the spark plugs were loose and I had to tighten them (courtesy of my mechanic) and on cylinder 5 there was a shortcut (everytime I squeezed my hand between the A/C compressor and engine block to reach the spark plug I got shocked . I looked up my distributor, the cap was in my opinion in quite a good shape (the pic is at the beginning of this topic) like it should be for 36k miles, however the rotor showed a little bit of burn at the very end of the metal plate (it was also changed for a new one). Wires were original (dated with 4-Q-65 PACKARD RADIO TVRS LR), they were not brittle, anyways I changed them for new silicon ones from AC DELCO Professional - and it did not have any impact on the engine rpm or anything else. Sparkplugs were, possibly, also factory, AC 45S (picture below, the first two are as they came out from the engine, third one I just cleaned for commparison). The mechanic put in before new ones - AC 45 RS (dunno if that's bad or good). Old orignial are a) points mechanics, b) condenser, c) coil, d) wire coil-distributor, e) vacuum can.
> I even readjusted all the levers and float to factory settings - nothing helped.
> 
> Maybe you can come up with some new thoughts


Here's a "new thought" for you, take a good look at the photos you posted of your carb, and then take a look at these images I found of the bottom of a Rochester 2GV. Notice anything different?


















Yes, that's right - neither image has holes in the throttle plates like your carb does. It sure looks like that some point, "someone" has "messed with" that carb and either drilled those plates and made the holes much larger or changed the plates out completely. Some models of Rochester 2-jets have idle air bleeds in the throttle plates, but they aren't nearly that large. I'm betting that's the source of your problem.

Those holes are what is letting your carb pass too much air and are probably why it still runs and idles too high even with the idle speed screw backed all the way out. When I saw that photo of your carb, I noticed the different colored spot on the throttle shaft in one of the bores which made me suspect that area had been messed with before. Take a flat bladed screwdriver and see if you can remove the screws without having to grind on them or 'force' them past the area where they usually are staked over. If they come out without too much resistance, that's another clue that they've been removed at some point previously.

Are there such things as salvage yards / wrecking yards where you are? It would be worth trying to find a 'donor' carburetor that you could get the plates out of, or perhaps even use the entire throttle body from.

Bear


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## Maczuga (Sep 16, 2020)

Gonna look that up in my manual first, at least I could weld that up and or plug temporary. This carb has also one special feature I did not mention before - a cooling valve atop the wall between both bores, right below the airhorn. As I remember from the manual this was used in accordance to the factory installed a/c. It prevented the fuel from varporization, thus speaking of percolation. How both things are related I dunno but there it is. It is stated that before I am going to fine adjust the idling I should first shut this ventilation valve. However I tried many times to keep it shut but it never changed anything what makes me believe that this valve was just some gadget. I‘ll try to post some pictures of it along with the manual schemes.


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## Maczuga (Sep 16, 2020)

Found another pick of the throttle body with exact the same drills that apperas to be NOS:








source: Contato - Grupo Lasse's


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## BearGFR (Aug 25, 2008)

Here's the thing. We already know that you've got a problem with not being able to set the idle low enough even with your idle speed screw backed all the way out. The way that screw works, is to control air flow through the carburetor at idle. More air = more rpm, less air = less rpm. That's just how it works. If you can't "slow it down" using that screw, then >it has to be< that the engine is getting air flow from some other source than what passes around the edges of the throttle plates under control of that screw. There's just no other possible cause for an uncontrollably high idle rpm other than too much air flow, so that means we have to start looking for other pathways that the engine can be getting air at idle that can't be controlled by the idle speed screw. The obvious possibilities are a vacuum leak somewhere, poorly fitting/sealing throttle plates, extreme wear around the throttle shaft that's leaking air at the points where it passes through the throttle body, idle air passages that are too large, or (let's hope not) a crack in the intake or in the carb body. if the problem persists with _every_ vacuum connection disconnected and sealed, and you haven't identified a leak around the points where the intake manifold joins to the heads, or around the ends of the throttle shaft, then the air source pretty much has to be somewhere in the carb itself.

Easy test: Pull the carb, put some sticky tape over those holes (on the top side), and try it. Not a permanent fix, but still an easy test.

Also see if you can remove or at least loosen those throttle plate screws without having to "deal with" the staking on the ends opposite the screw heads. If you can, then you know they've been removed before. Even if you can only loosen them a little, it should be enough for you to hold the throttle linkage shut with your hand and then re-tighten the screws. That will make sure they're centered in the bores and sealing as good as they can.

Bear


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## Maczuga (Sep 16, 2020)

Okay guys,
I have come up with sth new.
As for the drills in the throttle plates, they appear to be stock as you can see in the figure below. Probably coworking with the ominous percolation valve due to the stock a/c.
But on the next one figure (underside of the throttle cluster) there's this choke channel, which is (so I think) a direct connection between the choke housing and the lowest level of the carb- just below the throttle plates and needles, thus speaking of short cutting the idle passages. What if the choke housing has a vacuum leak? That would mean that there's a huge amount of air just bypassing the whole carb, isn't it? I always thought that the choke consisted of a spring tension located inside the choke housing, being activated by the heat dissipation of the intake manifold, thus changing the tension of the spring and so on the choke linkage and doing two things 1) opening choke throttle 2) moving the choke/fast idle cam with it's "high steps". Now there appears to be a third function with this channel running on the underside of the carburetor OR maybe it was made for adding a vacuum regulated choke instead of the springy one? However the channel is there and might leak, what do you think?
There still remains question why the leaking after I changed only the dwell 😅
P.s.: My manual unfortunately doesn't cover all basic knowledge (it's a mechanic's book) and sometimes it's hard for me to read only from figures and schemes


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## Maczuga (Sep 16, 2020)

I might want also add, that the idle screw works on cold to mid warm enigne, and than stops working when it get's hot.


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## BearGFR (Aug 25, 2008)

It's quite possible that once you corrected the dwell setting, it "unmasked" this problem that was being hidden by the dwell being so far off. I stand by my previous assertion: if your idle speed screw is ineffecitve, then you have air getting in from _somewhere_ it's not supposed to be. The challenge is finding it.


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## PontiacJim (Dec 29, 2012)

OK, here we go. I did a bunch of looking on the internet when I too saw those small holes drilled in the throttle blades. That is a "trick" in helping a carb to idle better when you have a big cam and rich carb, so thought the carb had been modified. Turned out that every carb that I looked at advertised as 1966 Pontiac had those same holes. So that is factory.

I told you about the choke housing. Was not sure which ones had the vacuum port or not. Apparently your does - so this can be a possible vacuum leak.

You mentioned that when you had gas squirting into the carb, you saw "wet" on the shaft. This means the throttle shaft is worn out. This will act the same as a vacuum leak.

That "T" thing with the spring found inside the carb is what I told you about, power piston. It sits on top of the "Power Piston" seen in your factory picture. That power piston adds more gas into the carb during wide open throttle. It has a spring inside that is pulled down to close off the piston when the engine has high vacuum. When you put the gas pedal to the floor and loose engine vacuum, the spring raised the power piston which then allows additional gasoline to be channeled into the engine. It basically will richen the air/fuel mixture when the engine needs more power. If it is not working correctly, or bad, it can become open and allow the additional gas into the intake and the engine will run rich. 

You have way too many issues going on all at once and you had a bad mechanic. I suggest you begin from the start. The spark plugs look old/bad. So here is what you should do - all new parts.

*Replace all 8 spark plugs. Gap the plugs .032-.035" and no more.
*New distributor cap - brass contacts, not cheap aluminum. Brass provides a better contact.
*New rotor. - brass, not flimsy flat steel contact. These are cheap and junk.
*New wires - which you said you have and installed. Make sure they are correctly placed in the cap - Pontiac firing order goes counter clockwise. What I like to do is put the timing mark on the balancer on "0" and with the rotor installed, place the cap into position, not locked down. Then note where the #1 spark plug wire is on the cap and lift up the cap and confirm that the rotor is pointing at that wire. It might be slightly off, but not much. If the rotor is pointing to the #6 spark plug wire, rotate the engine 1 more turn to bring the balancer mark back around to "0" and then check the rotor/cap firing position again.

Skip down below before installing any distributor parts EXCEPT the rotor. With the distributor cap off, and only the rotor installed, you are going to check how much stretch/wear the timing chain has - which can effect smooth running. Look below on how to do this.

*New set of points - get New Old Stock AC Delco brand from Ebay, or AC Delco from a parts store, or Echlin which is a NAPA store item here in the US.
*New condensor - do not use store bought as many of these are made in China and junk. Get NOS AC Delco from Ebay, or Echlin brand.
You can get points & condensor as 2 individual pieces, or as a 1 piece unit.
*Look at the cam in the distributor that the points rub on. You should have a small tube of grease that gets lightly applied to the points cam to help with wear on the points rubbing block that goes up against the dist. cam. Check to see if they look terribly worn, ie rounded. Move the rotor shaft side to side to see if it has a lot of play. Should not have much. It will go up and down a little, this is OK. Do the advance weights move freely? You can check the vacuum advance canister using a vacuum pump. If you apply vacuum to the canister, it should pull on the plate that the points attach to. You will see the plate move in one direction with vacuum applied, and go the other direction when vacuum is released.
*If all looks good, install points. Gap the points with a feeler gauge, .016" to begin with. Don't worry about the Dwell right now, but it should be 30 degrees.
*Check your wire connections on the coil to make sure they are tight and now worn or frayed. A bad or weak coil can create problems. You can test a coil, but not something I do - I just replace them with new.

*CHECKING TIMING CHAIN/GEARS FOR WEAR OR STRETCH.*
With only the rotor installed, and the balancer timing mark lined up with the "0", turn the crank to the right by hand BUT watch the rotor for movement. The second the rotor moves, stop. This removes the slack from the timing chain. Note where the balancer mark is. Now by hand, rotate the engine in the opposite direction until you see the rotor move, stop. Look at the balancer mark. The balancer mark should have only moved a little bit, no more than about 6 degrees. You can rotate the engine back again to confirm and stop the second the rotor moves. Again, no more than about 6 degrees. If the balancer timing mark moves more than 6 degrees, the chain/gears are worn out and need replacing.

If this is good, then go back up and continue installing your new points, condensor, cap, wires, etc..

Getting all this correct, will eliminate any ignition problems. Then you can work on the carb problem and begin fresh.


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## 1968gto421 (Mar 28, 2014)

If you need scarce parts, assemblies or even another carb, you wold want to contact the Carburetor Shop in Missouri, I learned about them years ago when I was in AACA and they helped people with obscure parts like carb for 1941 Packard. Jon,the owner, goes by _CarbKIng_ on the PY forum and is always very helpful. Here is the website:





__





THE CARBURETOR SHOP






www.thecarburetorshop.com





Best of luck with your elusive carb problems. You have good guys helping you like _Bear_ and _Jim_.


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## Maczuga (Sep 16, 2020)

Thank you very much,
I have recently looked up Mike’s Carb site for gaskets but I will check out yours too. I’ve noticed that getting nos parts like linkage could be quite a challenge. Speaking of replacement parts,
would you suggest any brands for the engine gaskets? I’ve got minor oil leaks at front and back axle, think those are simmerings leaking. 
Also I’d like to replace the suspension bushing (getting brittle) and I have minor problems with a) speedometer- it does not move for the first several miles, following some sticky movement from 30 mph up and like after driving another 5 miles it starts working fully until the next day... it freezes again. The odometer works without issues b) alternator does have sometimes problems with sufficient charging, would it help to replace it with some higher amps?

I wan’t to underline that I am trying to stay with this car as factory stock as possible. Lately I have even redone with my best possible accuracy the writing on the aircleaner housing (on the internet there are only cheap stickers) unlike the factory printing. 
Take a look if you like it









After repainting (via tampon printing)









Stay well!
Artur


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## Maczuga (Sep 16, 2020)

Hello mates !

It seems, that I have solved my problem with the engines rpm's. I have replaced my oem ignition coil and bought a second rpm reader, this time I took the timing pro light from Innova. As soon I hooked up the new timing light along with my old car meter it became clear that there was some technical issue on the meters themselves. When the old meter showed on hot idle around 1 100 rpms the Innova one told me 732 +/- 5  Believing that Innova is working properly, I must have kept detuning my engine all the time with the old meter and was way below 500 rpms back than, what caused all the engine hesitation and rough idle.
However I still have the issue with the jerking tranny. That is, as soon I put either Drive or Reverse, the whole car jerks into it. At this point I'd say that the fault lies inside the tranny and not the engine anymore. I can idle now at as low as steady 550 rpm (Innova) and still reach the 19+ inches of mercury of vacuum.
Do you have any clue what else I could do to prevent the jerking? Shifting trough the gears whilst driving is all good and soft.

Thanks,

Maczuga


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## PontiacJim (Dec 29, 2012)

Maczuga said:


> Hello mates !
> 
> It seems, that I have solved my problem with the engines rpm's. I have replaced my oem ignition coil and bought a second rpm reader, this time I took the timing pro light from Innova. As soon I hooked up the new timing light along with my old car meter it became clear that there was some technical issue on the meters themselves. When the old meter showed on hot idle around 1 100 rpms the Innova one told me 732 +/- 5  Believing that Innova is working properly, I must have kept detuning my engine all the time with the old meter and was way below 500 rpms back than, what caused all the engine hesitation and rough idle.
> However I still have the issue with the jerking tranny. That is, as soon I put either Drive or Reverse, the whole car jerks into it. At this point I'd say that the fault lies inside the tranny and not the engine anymore. I can idle now at as low as steady 550 rpm (Innova) and still reach the 19+ inches of mercury of vacuum.
> ...


Glad you got that solved.

The jerking could be normal, or it could be that a shift improver kit has been installed in the transmission. Make sure all your U-joints are good and check the play in the rear end - pinion gear/ring gear may have a lot of slop due to age & wear.

I don't know if a vacuum line, or the vacuum modulator could be suspect - I would check these. I believe there is a screw adjustment on the modulator and you can do a web search and see if this does anything to soften the trans.

Found this:

"One may also ask, what are the symptoms of a bad vacuum modulator? When you have a bad modulator valve, some or all of the following symptoms will begin to manifest themselves:

A whistling sound (from a leaking diaphragm)
White smoke coming out of the exhaust (from a leaking diaphragm)
Early or late shifting.
*Hard shifts (usually causing the car to jerk)*
A rough idle."


Hopefully others more qualified will provide some checks you can do.


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## Maczuga (Sep 16, 2020)

Hmm, as far I understand the modulator can only change the shifting points of the gears 1-3. I have a friend who’s got a 1968 Bonneville with the 400 cu in engine and I believe also TH400 tranny. His car goes into D/R almost unnoticable. 
Do you think it would be safe for the engine to go further down with the idle rpm - below manufacturer specification to lower the vacuum?
I am at 650 rpms atm with tranny in Park, A/C still disconnected (no belt).


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## integrity6987 (May 10, 2017)

I'm in the "u-joint suspect" theory until checked-off. With car off and in-park, get under it and try to rotate the driveshaft (both directions). Rotation/play should be minimal. If not your u-joints/and or differential gears need attention.


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