# Rough Idle 68 GTO 400



## musclecar65 (Mar 17, 2017)

Hi, just bought this "High End" driver with a 400/350 Automatic/Air, 94,000 miles and among the many, many issues it has, one is a rough idle. Had the Quad rebuilt, installed Petronix Ignitor 1 and Flame Thrower coil, new manifold gaskets, all new vacuum hoses & Thermactor, plugs (gapped at .40), wires, cap and rebuilt the air cleaner - all restoration quality parts. Timed at 9 degrees TDC (steady), vacuum at 18psi (steady) at 650 rpms (erratic - wavers). Idle is still rough, puffs light smoke sometimes and think I hear periodic tapping on left bank. Open to suggestions, comments, criticisms and WTFs?... Thanks


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## PontiacJim (Dec 29, 2012)

musclecar65 said:


> Hi, just bought this "High End" driver with a 400/350 Automatic/Air, 94,000 miles and among the many, many issues it has, one is a rough idle. Had the Quad rebuilt, installed Petronix Ignitor 1 and Flame Thrower coil, new manifold gaskets, all new vacuum hoses & Thermactor, plugs (gapped at .40), wires, cap and rebuilt the air cleaner - all restoration quality parts. Timed at 9 degrees TDC (steady), vacuum at 18psi (steady) at 650 rpms (erratic - wavers). Idle is still rough, puffs light smoke sometimes and think I hear periodic tapping on left bank. Open to suggestions, comments, criticisms and WTFs?... Thanks


Did you eliminate the resistor wire to the coil and substitute with a 12V powered wire?

When AC is activated, do your idle RPM's increase? If not, maybe your idle speed is dropping below 650 with AC on and affecting idle smoothness.

Vacuum advance connected and working correctly?

94,000 miles - new timing gears/chain installed to replace the factory nylon coated cam gear? Sloppy chain/worn gears will affect timing and can cause idle to waiver.

Puffs light smoke sometimes - worn valve guides or valve seals, worn rings.

Periodic tapping on left bank - Have you checked the rocker arms to make sure none have backed off? Low oil pressure? Cam lobe going flat. Lifters worn out. Incorrect oil. Valve springs weak.


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## musclecar65 (Mar 17, 2017)

Man, that is quite a list. Resister wire , yes; Idle rough with and without AC on; vacuum advance - new and working; original chain and gears; going to do a compression check when pan gasket is replaced; correct oil but will have the rest checked for wear. Thank you, great suggestions!


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## armyadarkness (Dec 7, 2020)

musclecar65 said:


> Hi, just bought this "High End" driver with a 400/350 Automatic/Air, 94,000 miles and among the many, many issues it has, one is a rough idle. Had the Quad rebuilt, installed Petronix Ignitor 1 and Flame Thrower coil, new manifold gaskets, all new vacuum hoses & Thermactor, plugs (gapped at .40), wires, cap and rebuilt the air cleaner - all restoration quality parts. Timed at 9 degrees TDC (steady), vacuum at 18psi (steady) at 650 rpms (erratic - wavers). Idle is still rough, puffs light smoke sometimes and think I hear periodic tapping on left bank. Open to suggestions, comments, criticisms and WTFs?... Thanks


Who did the work? Was it done before you bought it, or after?

If theres a big cam, then 9 degrees of advance is a sure fire way to have a rough idle.

If the plugs are gapped at 40, make sure your wires are routed well and in tip-top shape. Spark will take the course of least resistance, so the bigger your gap, the bigger your chances are of arcing and misfire.


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## kevin1727 (Aug 4, 2021)

armyadarkness said:


> .......
> If theres a big cam, then 9 degrees of advance is a sure fire way to have a rough idle.
> .......


What is considered a "big" cam? 
Is it mainly one of these things? 
Valve Lift
Duration
LSA
Valve Timing


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## armyadarkness (Dec 7, 2020)

kevin1727 said:


> What is considered a "big" cam?
> Is it mainly one of these things?
> Valve Lift
> Duration
> ...


Im not the most qualified to answer that, but lift is usually the number one characteristic to determine whether or not it's radical... Then duration. All of the other specifications matter, but always because they're working together and complimenting each other.

No reason to have high lift on a mild cam.


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## geeteeohguy (Feb 2, 2008)

Does the engine speed up or slow down when you spray a small amount of carb cleaner down the carb? This is the quickest way to determine if it's too lean with a vacuum leak.


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## BearGFR (Aug 25, 2008)

> Idle is still rough, puffs light smoke sometimes and think I hear periodic tapping on left bank. Open to suggestions, comments, criticisms and WTFs?... Thanks


Rough idle MAY be normal if the engine has a "big" cam. If you can provide video with sound of it running, that would help.

"Big" is a relative term, and also a cam that would be somewhat rowdy in a small displacement engine can be quite tame in a large displacement engine.
Do you have the specs on the cam you have?
Specs: Valve lift, intake and exhaust duration (at 0.050 tappet lift is preferable, but also can use "advertised" duration numbers if the 0.050 numbers aren't available), and LSA (lobe separation angle)

You mentioned that it idles at 650 and has 18 inches of vacuum there, so the cam probably isn't TOO rowdy.

You also mentioned the possibility of "periodic tapping". That could be one or more rocker arms out of adjustment which can play havoc with valve timing, vacuum, and idle. The factory Pontiac rocker system was non-adjustable. You were supposed to torque all the rocker nuts down onto the bottlenecks on the studs (as I recall the torque value was 25 lb ft) and leave them alone. Sometimes people who don't know this about Pontiacs will try to adjust the rockers like they do a ::cough:: chevy, and in short order the nuts will back off and things will get noisy and poor running.
But, if anything in the valve train (cam, lifters, rockers, pushrods) have been replaced with aftermarket parts, then "all bets are off" and "poly-lock" rocker nuts are required and proper adjustment is mandatory.

Another possible cause of a poor idle: vacuum leak. Examine all your vacuum hoses and connections (including the one on the transmission vacuum modulator if it's an auto) for splits, cracks, leaks, disconnected hoses, etc. 

Also, ignition timing. What matters with timing is how much TOTAL advance (tested with distributor vacuum disconnected and plugged) you have at around 2500 RPM (or wherever the advance is "all in"). Every distributor will have "so much" mechanical advance built in, so the overall total will be how much is in the distributor, plus the initial. The initial setting by itself doesn't matter all that much. What matters is the overall total. 


Cheers,
Bear


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## musclecar65 (Mar 17, 2017)

Thanks to all for your suggestions and advice! The engine is stock and original. All vacuum lines were replaced. The rough idle was there before I performed all of the restoration (myself). I've tried setting the timing from 12 to 6; it runs faster at 12 but the specs say 9. Gapped the plugs at .40 as the consensus is that it's better with the Ignitor and Flamethrower coil, added new high quality wires. Tried adjusting the carb every which way but loose, just can't get the tach to steady up. Oil gauge moves up and down with engine rev much more than it did before I changed the sludge to oil and a new filter so that may be affecting the valves. New pump, pickup and pan gasket coming the 25th. Thanks again, guys


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## armyadarkness (Dec 7, 2020)

Forget the spec, if the engine liked 12 degrees of timing, then I would set it at 12! Then reset my mixtures for the highest vacuum, then drop the idle.

BTW, you mentioned a rebuilt Qjet. Who rebuilt it? The market is FLOODED with HORRENDOUS Qjet rebuilds. Anything/ most stuff from Pep Boys, Advanced, Autozone, ebay, Amazon... is CRAP!

And where will you notice that crap most? At idle. Poor rebuilds often have badly worn throttle blade shaft bushings... and they dont idle for poo!


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## geeteeohguy (Feb 2, 2008)

Again, surging, unstable idle is a classic sign of a vacuum leak. Drizzle some carb cleaner down the primaries of the carb and see if it speeds up.


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## PontiacJim (Dec 29, 2012)

musclecar65 said:


> Thanks to all for your suggestions and advice! The engine is stock and original. All vacuum lines were replaced. The rough idle was there before I performed all of the restoration (myself). I've tried setting the timing from 12 to 6; it runs faster at 12 but the specs say 9. Gapped the plugs at .40 as the consensus is that it's better with the Ignitor and Flamethrower coil, added new high quality wires. Tried adjusting the carb every which way but loose, just can't get the tach to steady up. Oil gauge moves up and down with engine rev much more than it did before I changed the sludge to oil and a new filter so that may be affecting the valves. New pump, pickup and pan gasket coming the 25th. Thanks again, guys


The Pertronix conversion seemed to me to work better on 12V versus the resistor wire - even though the instructions may say so. I had the resistor wire on my '68 Lemans along with a 12V power wire as a "jumper" hooked to a toggle switch. I noticed a difference in the running of the engine when I flipped the 12V wire to over-ride the resistor wire.

You are going to have to pull the engine for the oil pump & pan gasket - unless your mechanic has small hands and is a contortionist. The oil pressure will change with engine RPM from idle to around 2,600 RPM range where it should be constant - and depending on engine temp and oil type/thickness/filter type. If the engine still has the original timing chain/gears, the nylon coating on the gears can eventually errode and break off and go to the pan and plug up the oil pump pick-up screen which would cause poor oil pressures.

Easier to do a new chain/timing gear with the engine out.

With 94,000 miles, it is a good bet that the engine is worn out and in need of a rebuild - especially if it was not taken care of with regular oil/filter changes, tune-ups, and driven reasonably.


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## AV68 (Aug 18, 2021)

If you did the Pertronix upgrade to the stock distributor you definitely want 12v to it not through the resistor wire and you could try closing the spark plug gap a bit, my stock 68 had both vacuum advance and vacuum retard don't know if they were all like that or just Ca cars, look for vacuum leaks on both 
I used a new Pertronix distributor in my 68 400 and definitely needed to run a new power source to it and it liked the smaller gap on the plugs better as well 
Change the timing gears when you have it out like Jim said pic below is stock 54 year old cam gear with 99k miles, surprisingly the engine ran smooth as silk I had no idea it was that bad


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## AV68 (Aug 18, 2021)




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## geeteeohguy (Feb 2, 2008)

So the OP has a destroyed timing chain? I went back through all the posts and still can't seem to find it.


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## AV68 (Aug 18, 2021)

No the OP said he was going to replace the oil pump and pan gasket, Jim suggested doing the timing gears while he had it out


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## PontiacJim (Dec 29, 2012)

AV68 said:


> View attachment 158001


1968 engines, not just CA cars. Pontiac issued a bulletin, Service Bulletins 68-T-2 (dated 10/16/67) and 68-T-2A (dated 1/4/68), to correct/re-route the vacuum lines to eliminate the "retard" feature. My '68 Lemans with factory 350CI had that distributor. It would have been connected to the Thermostatic Vacuum Switch (TVS) on the intake. There should be 5-ports and the vacuum lines on the 1968 cars attach like this, Inputs: Carb Advance, Carb Retard, Manifold Vacuum. Outputs: Distributor Advance, Distributor Retard. 1969 V8 with automatic, some early RA III cars, and 1971 455 HO engines used a 3-port TVS.


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## BearGFR (Aug 25, 2008)

musclecar65 said:


> ... I've tried setting the timing from 12 to 6; it runs faster at 12 but the specs say 9. ....


Again, initial timing setting is irrelevant, especially if anything in the ignition system/distributor is non-stock or has been modified, possibly by a previous owner. What matters is the total after all the mechanical advance is "in" (usually at 2500+ rpm "or so"). Use a "dial back" timing light that can measure total advance, disconnect and plug the vacuum line to the distributor, have a friend slowly increase RPM while you watch with the light until the advance stops increasing. Use the light to see what the total amount is. Pontiacs with open chamber iron heads "generally" will perform best with about 34-35 degrees total, all in by 2500 rpm. 

Here's what to do with the information:
Let's say that right now you have your initial set at 9 (that's the last setting I remember you mentioning).
You go through the above process, and find out that you have 30 degrees total at, oh let's say ... 2400 RPM, and it doesn't advance any more after that.
That tells you that in order to get 34 degrees total, you need to add 4 more initial - so you'd set your initial to 13.
After you do that, check it again at RPM to verify it's now at 34.

NOW you know that to get 34 total, you have to set your initial to 13 --- so you don't have to measure it at RPM any more --- unless you make changes to the distributor.
Reconnect your vacuum, take it out for a spin and see if it's "happier".

Before I switched to the ignition I have now (Progression Ignition HEI), the mechanical HEI I was running "had" 20 degrees of travel in it. I'm running aluminum heads that from experience I know perform best with 35 degrees total, so that means with that system I had to set my initial to 15. 15+20=35.

(The Progression Ignition HEI I have now is fully programmable - with my phone. So far, I like it a lot)

Bear


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## geeteeohguy (Feb 2, 2008)

I wonder if the OP sprayed some juice down the carb yet to rule out or confirm a vacuum leak. I think if he did, he might be surprised.


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## musclecar65 (Mar 17, 2017)

Lot's of great advice, tooo much to absorb at one time but, I have 12V to the Pertronix, eliminated the vac and carb retard by jumping the two on the thermactor like the factory spec said. Reset timing to 12. Will need a new timing light to check total timing as mine is 50 years old. Will replace timing gears and chain as a precaution although timing doesn't change with/without vac and is steady.

Idle is very good now, although moves forward without brake, until it warms up - then idles down a little and the engine gets uneven although the car hardly shakes. Once you hit the throttle it's smooth as silk up to 100. Uses more gas though, can hear backfiring in mufflers when decelerating, and strong emissions smell when stopped. Sometimes does that hard to turnover routine when the key is turned, like timing is off. Getting better although the water pump just started leaking  Getting ready to spritz carb cleaner down carb to check vacuum.

Thanks to all for your suggestions!


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## PontiacJim (Dec 29, 2012)

musclecar65 said:


> Lot's of great advice, tooo much to absorb at one time but, I have 12V to the Pertronix, eliminated the vac and carb retard by jumping the two on the thermactor like the factory spec said. Reset timing to 12. Will need a new timing light to check total timing as mine is 50 years old. Will replace timing gears and chain as a precaution although timing doesn't change with/without vac and is steady.
> 
> Idle is very good now, although moves forward without brake, until it warms up - then idles down a little and the engine gets uneven although the car hardly shakes. Once you hit the throttle it's smooth as silk up to 100. Uses more gas though, can hear backfiring in mufflers when decelerating, and strong emissions smell when stopped. Sometimes does that hard to turnover routine when the key is turned, like timing is off. Getting better although the water pump just started leaking  Getting ready to spritz carb cleaner down carb to check vacuum.
> 
> Thanks to all for your suggestions!


Sounds good. You can test the gears/chain by pulling the cap and observing the rotor movement. Put a mark on the balancer at your 0 mark and then gently, by hand, rotate the crank in one direction until the rotor just moves - this will take any slop out of the chain/gears. Take note of your mark. Now rotate the opposite direction until the rotor moves - stop. Look at your mark. How many degrees did it move? Now go back the opposite direction and stop the minute your rotor moves, this will confirm the degree movement as your mark should be right back to the 0. You can do this another time if you want to confirm. Should not have any more than about 4-5 degrees of movement.

Hard/labored starts is typically a sign of too much advance, as well as backfiring BUT backfiring can also indicate a leak in the exhaust manifolds as it sucks in additional air with the unburned fuel being sent out the pipes when you let off the gas pedal.

It is possible that your balancer is off. The rubber bond on them can deteriorate causing the out ring to slip - then timing is off and you are adjusting timing to that difference.

With the timing advanced, your RPM's have increased? - which can be normal. You can most likely adjust your idle speed OR your high idle cam screw which works with the operation of the choke. Screw it out to lower the increase in idle, either cold idle or warm idle, so the car won't be creeping forward and need the additional brake. Some creep may be normal as well.

Do 1 adjustment at a time. Start with looking at your balancer's condition. Then check the chain/gear play as a lot of play can affect timing. Then check timing - Initial, maximum advance (vacuum disconnected) at "X" RPM where no more advance takes place, and then what the vacuum advance adds to this. The vacuum advance is important for helping the engine to run cooler and improve gas mileage.


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## musclecar65 (Mar 17, 2017)

geeteeohguy said:


> I wonder if the OP sprayed some juice down the carb yet to rule out or confirm a vacuum leak. I think if he did, he might be surprised.


Sprayed some juice down the carb and it idled down. With all new hoses, vac advance, thermactor, etc. and a steady vacuum at 17 and steady timing I'm at a loss... wish it was, then I could fix it.


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## geeteeohguy (Feb 2, 2008)

musclecar65 said:


> Sprayed some juice down the carb and it idled down. With all new hoses, vac advance, thermactor, etc. and a steady vacuum at 17 and steady timing I'm at a loss... wish it was, then I could fix it.


You are not too lean then. Not a vacuum leak. Explain exactly what is happening. We can figure it out.


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## musclecar65 (Mar 17, 2017)

PontiacJim said:


> The Pertronix conversion seemed to me to work better on 12V versus the resistor wire - even though the instructions may say so. I had the resistor wire on my '68 Lemans along with a 12V power wire as a "jumper" hooked to a toggle switch. I noticed a difference in the running of the engine when I flipped the 12V wire to over-ride the resistor wire.
> 
> You are going to have to pull the engine for the oil pump & pan gasket - unless your mechanic has small hands and is a contortionist. The oil pressure will change with engine RPM from idle to around 2,600 RPM range where it should be constant - and depending on engine temp and oil type/thickness/filter type. If the engine still has the original timing chain/gears, the nylon coating on the gears can eventually errode and break off and go to the pan and plug up the oil pump pick-up screen which would cause poor oil pressures.
> 
> ...


Hi PontiacJim, just a follow up. Did as you suggested and replaced the timing chain and gears although the mechanic said the chain was tight. Compression was within limits and valves quiet so didn't rebuild. Correct carb adjustment on the rebuilt seems to have done the trick on idle. Also replaced the rear main which it turns out was leaking. Thanks for the suggestions


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