# Goofy tuning set-up (high timing and extra idle fuel) - potentially jets?



## 1967pontiac400 (Dec 10, 2020)

Hi All,

I have a somewhat complex issue I'm hoping to get some inpout on (or perhaps simple for some of you Pontiac veterans, lol). 

Background: 69 Pontiac 400 (non HO or RA) with a 65 tripower, running 100 octane, in a 67 GTO.

At the moment I have the car running relatively well. It starts up fine, idles okay and drives well. However, it is currently set-up with more than standard timing, I forget now where I set it, but it's at least 10-15 degrees more than is recommended. To ensure there's no detonation, I'm running the extra octane to help. 

Initially, the car would not idle well at all and has next to no vacuum. I evantually was able to figure out that the end carbs never were treated with DAG and had vac leaks. Once I completed that project and re-did the initial set-up we got the car to where it is now. Idling fine, but with lots of timing and extra fuel in the idle circuit. This setting on idle is rather rich, I forget how many turns from the recommended 2.5 turn back-out I set it to, but I think it's at least one full turn beyond that. The idle is rich enough where the air is pretty stinky when idling. 

My thinking is, with all of this, is that the jets might just be incorrect for my setup. They are the last thing in the tripower unit that I have either not replaced or overhauled at this point. The tripower was purchased at a swap meet in the late 70s and I've noticed and experienced that it was far from a "ready to run" tripower. So it pretty resonable to conclue that the jets are off spec. I can't image that the 400 I have in the car (non HO or RA) would need much more fuel than the standard 389 from 65.

It has been recommended that we toss in a .063 in the center and .068s on the end carbs. Before I went ahead and did that, I wanted to get some input to make sure there's nothing else I might be missing in order to get the timing and idle mix setting in a closer to standard place while maintaining healthy vacuum.

Thanks as always!


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## BearGFR (Aug 25, 2008)

There have been many many threads on here discussing the subject of ignition timing. You might avail yourself of the search tool to find them.

However, initial timing is insignificant and certainly can't be classified as a 'problem' if it so happens that your car seems to "like" more. There's only one reason that the factory spec is 6 degrees, and that's because they already knew two pieces of very important information: 1) how much total advance at RPM was 'right' for their engines and 2) how much mechanical travel was present in the factory distributors they put on the cars. Because of that, they knew that 6 degrees initial, when combined with the amount of travel they already knew was present in the distributor, _would result in_ the amount of total timing they wanted. Hence the spec of 6 degrees initial, not because there was anything magic about that number, but because that initial setting would put "everything else" where they wanted it to be and it's a lot easier to set timing on an engine with it sitting there, sedately idling, than it is setting total timing while it's buzzing along at 3500 RPM. That can be a little unnerving to do if you're not used to doing it.

Now, if anything on the car has been modified from dirt stock - different distributor, different heads, different cam, different fuel system/carburetion.... then all bets are off. That 6 degrees initial may no longer result in the amount of total that the car needs and likes - and that's the number that really matters, not the initial setting.

So, if your car is running well, you're happy with it, and it's not getting into detonation --- leave it alone and keep putting timing where it's happy. Don't worry that it's different from 'factory'.

I run my '69 at 15 degrees initial --- because I know from testing that it _likes_ 35 degrees total the best, and I also know that the HEI ignition that I'm running on it now only has 20 degrees of travel "baked in". Ergo, 15+20=35.

Cheers,

Bear


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## 1967pontiac400 (Dec 10, 2020)

Thanks Bear, I appreciate the recommendations and response. 

I have read many threads and learned a lot. What I have been unable to gather from the previous thread though, is if too small of jets could net a scenerio where extra advance and a richer idle fuel mix could offset that. I suppose what I'm asking is, if I'm masking a problem.

I suppose putting in the recommended jets and going from there might be the best path since I have no idea what jets are in the car now.


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## BearGFR (Aug 25, 2008)

It's possible I 'spose. Lean mixtures burn more slowly and will need more lead in order to fight against overheating. That's the reason for the vacuum advance can. It adds more timing during light load, part throttle conditions to offset the leaner mixtures that tend to happen. They burn slower, and if they're still burning when the exhaust valve opens it leads to overheating. The object is to make sure all the burn is finished before that valve opens.

Lean mixtures also burn hotter and are more likely to get you into detonation. 

I tell you, putting a good quality Air/Fuel meter in my car was one of the best decisions I made. Considering all the problems I had when I decided to go with a 'bigger' carb, trying to get it tuned right without one would have been hopeless.

There are "portable" meters, Like the LM-2, and I bought one of those first. Later though, I put in an AEM UEGO digital full time panel mounted gauge, and it really saved my bacon.

There's nothing like knowing for certain where your fuel mixtures are, in all modes of operation.

Bear


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## Kingo (Jan 5, 2021)

Make sure your fuel pressure to carbs is below 6 pounds. Mine ran rich and the fuel pressure was the cause. Gave me all sorts of tuning fits….


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## Lemans guy (Oct 14, 2014)

67 I second everything Bear said. Now let’s talk about your rich idle and jets.

jets don’t come on until about 2500 RPM. They are not engaged at all at idle.

in fact you could unscrew and plug the jets and drive around under @about 2500 RPM and it would drive fine. On the idle circuit the pie are two idle passages for each mixture screw, plus a transfer slot. The mixture do not…….get this……mix anything!

They just provide a mixture of fuel and air. On most carbs turning the idle mixture screw in or clockwise delivers less of the “Mixture” and turning the screw out or counter clockwise delivers more of the mixture.

The transfer slot is best set with the carb off and upside down and turning the throttle screw until just a square portion of the slot shows below the throttle plate. This ensure that when the throttle plate is slowly opened by your gas pedal fuel mixture flows thru the transfer slot to aid the idle passages.

you are too rich. Many things can cause that, but not jets at idle or under 2500 Rpm.

often the transfer slot is to blame either it is way too open and delivering way too much fuel Mixture at idle…..or it is slammed shut and you have to open the mixture screws too much to get it to idle.

I would flip the center carb over and get the transfer slot right. Then try to set the mixture screws and read up on all the timing threads. These engines like a lot of idle timing, 15 to 30 degrees but it has to be done properly or you will blow up the engine


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## 1967pontiac400 (Dec 10, 2020)

Kingo said:


> Make sure your fuel pressure to carbs is below 6 pounds. Mine ran rich and the fuel pressure was the cause. Gave me all sorts of tuning fits….


Good call, this was a previous problem causing flooding. There's a regulator on there now holding 3.5 lbs.


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## 1967pontiac400 (Dec 10, 2020)

Lemans guy said:


> 67 I second everything Bear said. Now let’s talk about your rich idle and jets.
> 
> jets don’t come on until about 2500 RPM. They are not engaged at all at idle.
> 
> ...


I did not know about the transfer slots, I guess it's time to take the center off and give that a look see!


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## 1967pontiac400 (Dec 10, 2020)

Welp, I had the wrong carb for the car. It turned out to be a chevy carb, not a correct 1 1/4 pontiac center. 

I've since tracked one down, restored it and have her back up and running. 

Good idle speed, healthy sound, rock steady tach needle and all else look great. There's just ONE weird thing...

There's a bit of a stumble running on the center carb, I took the center back apart and noticed that the squirt was weak. Plugged the sectin where it draws from the float bowl (since the slot fills this up anyway) and the issues is MOSTLY resolved. However there's still some stumble at lower speeds when applying throttle. I also noticed that adjusting the idle mixture screws do nothing to the vac gauge - I'm not sure what I'm missing, I'm sure it's something simple.


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## Lemans guy (Oct 14, 2014)

So it sounds like a lean stumble. If it stumbles at speed, like going from 30 to 40 MPH I would look for a weak accelerator pump squirt. If it stumbles from stop, say off a traffic light I would suspect the transfer slot normally. But I am sure that you squared that as previously discussed.

You may have air leaking past throttle blades at idle ( if transfer slot is right) that won’t allow the mixture screws to change the vacumn. It does not take much.

I am sure that you have tried blocking off the air inlets on both outside carbs at idle and then trying the center mixture screws. If that won’t allow the mixture screws to function then maybe the throttle plate needs to be closed tighter. Or you have a vacumn leak somewhere.

around the carb mounting gaskets or shafts or somewhere. Check for that. Even clamp off any vacumn lines from the intake. Brake booster, trans module, dash controls and then try to adjust the mixture. if it now works release them one at a time until it does not to find and trace the vacumn leak.

Small vacumn eaks can effect idle. Try this stuff first. I use a smoke machine to find difficult vac leaks. All auto shops have them. So if nothing is working. Take it to any garage and see if they will smoke it while you wait. They may charge you a little but if it locates the leak it helps.

Of course there are all the hose to ear, and squirting, Wd-40, and using propane and watching for speed up of RPM’s, even blowing cigar smoke Into the intake. …..but none of those are that good. Smoke from a smoke machine is best on difficult vac leaks.

some though are big and you can even hear them….

remember to .check your accelerator set up first…..


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## 1967pontiac400 (Dec 10, 2020)

So one of my throttle blade assemblies has transfer slots and the other does not....hmm. 

Should blades be fully closed at low idle (choke open)?


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## Lemans guy (Oct 14, 2014)

When hot, throttle blades should be closed at idle, with middle carb being set with a squared transfer slot. If you have a transfer slot in an outer carb that is for transitioning from idle to moving, but that slot lets in a mixture of air and fuel….so sorry not sure on a Tri power.

But my understanding they only idle off the center carb, not the outer carbs. So if you have one outer with a transfer slot and one without I would say one is wrong. Probably the one with the transfer slot. As when those normally would come on there would be no need for a transfer slot as the center carb would already be pretty well open. And a transfer slot would do nothing.

it may be why you cannot tune the mixture, try closing that throttle off rightnd closing the transfer slot and see how it effects vacumn.


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## Lemans guy (Oct 14, 2014)

Choke open when at hot idle….yes


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## 1967pontiac400 (Dec 10, 2020)

Sorry, let me clarify...I have no issues with the outer carbs, that's not on the checklist for this issue, I have already verified there are no issues with vac leak there, incorrect parts, etc.

I have two different base plates for my center carb, one has transfer slots and the other does not. I was able to run the car on both, but it seems that the new base plate (w/o transfer slots) was possibly idling off the idle circuit and that's the plate where the idle mix screws are doing nothing. 

I'm leaning towards putting the old base plate (w/ transfer slots) onto the new carb and seeing where I land. I just hate taking this thing on and off so many times. The tripower is really making me work for it. lol.


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## Lemans guy (Oct 14, 2014)

Yes, a correct idle circuit has an idle feed hole and a transfer slot. Both provide fuel and air at idle. The “mixture” screw does not reall mix anything.b it it changes the volume of the mixture.

if the transfer slot is wrong then the idle mixture can be messed up. I would say that you need the one with the transfer slot, preferably squared. Now you likely have bog when opening the throttle as you get more air from the throttle plate but no offset fuel mixture from the transfer slot to counter that rush of lean air.

especially if you gradually go down on the pedal, the transfer slot covers the transition off idle, at speed the slot is already away from the throttle plate and the accelerator pump squirt provides the extra fuel needed to cover the rush of air.


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## BearGFR (Aug 25, 2008)

Suggestion: get/make some block off plates and remove the outer carbs, leaving only the center carb. Get it idling and running right on just the center carb, which should be the only one that has idle circuits at all. Yeah, it's not going to make power running that way but I'm hearing that's not where your issues are. If the end carbs are leaking air, either because the shaft bushings are worn out because the throttle plates aren't completely sealing, that's going to cause problems. Once you've got it right on one carb, put the end ones back on. If your problems return, you'll know it's the end carbs that are the cause. 

Bear


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## 1967pontiac400 (Dec 10, 2020)

I have already narrowed the issues down to the center carb, and swapped the base plate for the one I have with the transfer slots. Only issue now is i broke off my stupid hollow stud, need to wait for a new one from ames now 😫.

The end carbs have been rebuilt, DAG sealed, etc. But I appreciate the suggestion, I can't tell you how many months of trial, error and research was needed before I got those to work correctly.


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## PontiacJim (Dec 29, 2012)

My opinion. First, stock cam or aftermarket? What is your vacuum reading at idle? 

Generally, when you get a stumble when you give the engine the gas, it means the engine is gulping more air than fuel. That's where the gas nozzles/squirters in the carb's bores come into play - along with the accelerator pump.

If you look down into the carb with a flashlight and choke wide open, pull the throttle arm open just as if you were putting the gas pedal to the floor. You should see a good solid stream of gas shoot into the carb from each nozzle, left & right. If the gas stream is not strong and shoots for a bit the minute you pull the throttle open, then there is a problem. If it is a weak stream or quits rather quickly, then you may not be getting enough gas when needed when the carb is being opened and sucking in air.

You need to know that the accelerator pump is ethanol friendly and the correct one for your carb's ID number.

If you have a bigger cam that needs more fuel, the carb has a power valve and a spring that works off of vacuum. If the vacuum signal is less than factory, this can cause the power valve to be slightly open/open dumping extra fuel into the engine when not needed.

You may have already viewed these, but here are a couple YouTube videos that may help with how the carb idle works and how to adjust for rough idle.











Check out this YouYube video. You can also buy these if you need a replacement.






If the engine is surging while going down the road, make sure you do not have too much advance coming from the timing - generally too much vacuum advance being added to an initial timing that is already more than the stock 9 degrees BTDC. So it would be good to know all the numbers related to your timing, Initial, mechanical weights and at what RPM maximum advance is reached, and how many more degrees does the vacuum advance add to the total mechanical advance.

Surging could be a vacuum leak somewhere. It could also be too high of a float level with gas spilling into the carb while driving. A poor sealing needle and seat could allow the bowl to over fill, same as too high of a float setting - gas spills into the carb while driving.

So just a few more suggestions to check since you have isolated it to the center carb.


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## Scott06 (May 6, 2020)

1967pontiac400 said:


> I have already narrowed the issues down to the center carb, and swapped the base plate for the one I have with the transfer slots. Only issue now is i broke off my stupid hollow stud, need to wait for a new one from ames now 😫.
> 
> The end carbs have been rebuilt, DAG sealed, etc. But I appreciate the suggestion, I can't tell you how many months of trial, error and research was needed before I got those to work correctly.


We must own the same tripower set up… i have just been through about 9 months of tuning and tweaking as well.

a couple of thoughts, apologies in advance if you already did or mentioned these as ruled out

is your heat riser blocked? I orginally blocked mine and found the idle terrible and long warm ups. i pulled the block Offs and it runs and warms up much better.

i installed an AFR meter and found that invaluable. My idle ratio was periodically dropping as low as 12, tried needle and seat and different float level settings. Float level seems more critical on this set up than any other carb ive tuned before.

i also found fuel pressure at 3-3.5 is critical. I installed the pressure regulator made no diff, took it off… tried a new fuel pump end up with boosters weeping, reinstalled regulator, and then found the liquid filled guage pressure fluctuates drastically with temp. I set it cold and now ignore it weeping in eliminated and idle is much better, no longer burning my eyes. Seems to idle best 13.5 to 14 ish 

what is your vacuum at idle ? mine has never been great but i now have about 16” , on basically a stock engine (Spotts 068 HL cam). If you have a bigger cam with low vacum that can change some things.

did you drill out the idle tubes in venturi cluster? I think running a .033. Also the idle mix port in the throttle body needs to be opened up to .050” . Given how many rochester 2 jet models made who know what parts we are working with. If you cam is bigger typically folks drill these more as it will suck more air. Also folks drill small holes in throttle plates in some cases so throttle plate can flow enough air but still be closed enough to not expose idle transfer slots. this is mentioned in series of videos by guy who jim posted , i think this is one if the guys from pontiac tripower this info is on their website as well What to drill for what scenario.

one of the videos jim posted is on the accelerator pump circut. i got rid of. Big off idle bog by bending the linakge so the pump arm was preloaded so when throttle starts to move it is moving accel pump. There is also a bushing to fix slop in airhorn pump arm shaft if it wobbles. I may drill the venturi accel shot holes bigger to get more volume but i want to drive it some more.

make sure your front to rear carb linkage doesnt prevent end carbs from closing. Had this and accel pump rod on rear carb sticking the rear throttle blades from returning.

Im also running a lot of timing at idle. I think im at maybe 8 BTDC to set it with vacuum can disconnected, then run full manifold not ported vacuum. So all in at idle in the mid 20’s. When you hit the gas the vacuum drops out and you are working off mechanical only. I think I have total like 38 maybe more, might need to take out some mechanical due to the amount of intial i am running but im getting no pinging On 93 with lead substitute with stock compression.

jetting the guys at pontiac tripower recommended 63 center and 70’s front and rear. As mentioned you wont see fuel coming off the venturi until 2-2.5 k. I just went backed down to 61 s in the center and am getting the following AFRs 

13.5-14 at idle
15 or so at cruise (used to be out at 16-18)
13.5 ish under normal/ moderate acceleration 
12.0-12.5 at WOT

one last thing if you have low vacum due to a big cam or leak the power valve actuator maybe be lifting which i think would richen idle, but your mix screws are way out right? That would make me think the idle tubes in venturi aren’t big enough for you application, also why moving them doesn’t affect idle. is it possible your venturi is not out of a 65 tripower? The 2 jet was used on so many engines you may have a wrong venturi cluster. 

sorry for long winded post just also have had my center carb torn down about 20 times as well…


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## Scott06 (May 6, 2020)

Heres the link to idle circuit mods


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