1966 Hemi rough idle

[Jim Shook]

Looking for some ideas.
My 426 has a rough idle and shakes pretty good. It accelerates great
and has quite good power. Carbs are rebuilt and the ignition has been
upgraded to electronic. Can't find a vacuum leak anywhere, could an out
of adjusted lifter cause the problem? They don't sound too loud when idling,
but I'm not sure where to look anymore for the problem. Thanks for any thoughts!!!
Jim

 

[oldbee]

It's still a gas engine. Plugs,plug wires,coil? Need more info -hot/cold, all the time?Can be exasperating at times.

 

[hangn0ut]

Rebuilt doesnt mean right. Just because it dont take but a minute I would pull the idle screws on that back carb and hit where the screws were with compressed air. A quick blast,less than a second @ 40psi, ought to dislodge any crap in the idle circuit. Reassemble and start it up. See if there has been any change in the idle. That it runs well at higher rpm would tend to rule out ignition or valvetrain problems. But thats not an absolute. Make sure the carbs were set for float drop. To much drop can cause the needle to stick wide open. Oftentimes drop is not set or checked.

 

[Yatzee]

Looking for some ideas.
My 426 has a rough idle and shakes pretty good. It accelerates great
and has quite good power. Carbs are rebuilt and the ignition has been
upgraded to electronic. Can't find a vacuum leak anywhere, could an out
of adjusted lifter cause the problem? They don't sound too loud when idling,
but I'm not sure where to look anymore for the problem. Thanks for any thoughts!!!
Jim

If as you say, the ignition system is new and timing correct, and carbs are rebuilt properly and set up correctly ( I assume you have twin carbs because you didn't specifically say so) I would look to the valve train. Could be the lifters and valve springs are not compatible with the lift and duration of the cam. At higher operating rpm's, oil pressure to the lifters is higher giving them firmer pressure against the resistance of the valve springs so motor operation will seem smoother. If you have solids, then the adjustment of the rockers would be of great concern. You may have wiped a lobe or two on the cam but unfortunately the only way to tell is disassembly and possibly an investment of more cash. I had a similar issue when I put new heads on my 383 and found the valve springs were too stiff for the lifters to overcome at low rpm's. The valves were not opening consistently on all cylinders. I installed a new cam with matched lifters and checked with Lunati for max spring pressure for their combination. My existing valve springs were way too stiff but luckily the manufacturer of the heads sent me a new set of springs and I returned the originals. All is fine now, smooth idle, good power curve and no noise.

 

[Jim Shook]

It's still a gas engine. Plugs,plug wires,coil? Need more info -hot/cold, all the time?Can be exasperating at times.

Everything has been changed to date except for the coil. The problem is constant. hot
or cold.

 

[Jim Shook]

If as you say, the ignition system is new and timing correct, and carbs are rebuilt properly and set up correctly ( I assume you have twin carbs because you didn't specifically say so) I would look to the valve train. Could be the lifters and valve springs are not compatible with the lift and duration of the cam. At higher operating rpm's, oil pressure to the lifters is higher giving them firmer pressure against the resistance of the valve springs so motor operation will seem smoother. If you have solids, then the adjustment of the rockers would be of great concern. You may have wiped a lobe or two on the cam but unfortunately the only way to tell is disassembly and possibly an investment of more cash. I had a similar issue when I put new heads on my 383 and found the valve springs were too stiff for the lifters to overcome at low rpm's. The valves were not opening consistently on all cylinders. I installed a new cam with matched lifters and checked with Lunati for max spring pressure for their combination. My existing valve springs were way too stiff but luckily the manufacturer of the heads sent me a new set of springs and I returned the originals. All is fine now, smooth idle, good power curve and no noise.

Yes, it does have dual carbs, and i'm not sure of the role the second carb plays with the idle
circuit, if at all. She runs hard and accelerates awesome when you hit the throttle, but the idle
is pretty rough. I bought the car after it had been sitting in storage for 14 years, it does have
a solid lifter cam, so it maybe possible that there's valves out of adjustment.

 

[Jim Shook]

Rebuilt doesnt mean right. Just because it dont take but a minute I would pull the idle screws on that back carb and hit where the screws were with compressed air. A quick blast,less than a second @ 40psi, ought to dislodge any crap in the idle circuit. Reassemble and start it up. See if there has been any change in the idle. That it runs well at higher rpm would tend to rule out ignition or valvetrain problems. But thats not an absolute. Make sure the carbs were set for float drop. To much drop can cause the needle to stick wide open. Oftentimes drop is not set or checked.

Thanks for the ideas, I will follow up on your suggestions.
Jim

 

[lewtot184]

look to see if there's a piece of dirt in any of the idle jets or a clogged idle air bleed. a valve not seating fully can cause a rough idle but usually takes some rpm to smooth out.

 

[Yatzee]

Looking for some ideas.
My 426 has a rough idle and shakes pretty good. It accelerates great
and has quite good power. Carbs are rebuilt and the ignition has been
upgraded to electronic. Can't find a vacuum leak anywhere, could an out
of adjusted lifter cause the problem? They don't sound too loud when idling,
but I'm not sure where to look anymore for the problem. Thanks for any thoughts!!!
Jim

Then I'd go back to the carbs and their setup. Unless you have a completely divided intake manifold, one of the tricks we used to use, in days gone by, was remove one carb and use a block-off plate in it's place. Although your single carb may see small in CFM's it's more than adequate to maintain idle on a manifold with a common plenum. Set up the best mixture and smoothest idle you can get and then swap the other out and repeat. If yours is set up like a triple carb, only one carb handles the idle ( usually middle ) and the others acts as a secondary. Perhaps in your situation your running a bit rich and washing down the cylinder walls and plugs but you would know if you stood behind the car and inhaled or if you had an exhaust gas analyzer. I would assume there's something funky if you were to try and set idle with a pair of high CFM carbs because I would think there would be too much fuel atomization going into the intake or not enough vacuum for the pair to operate correctly. You really need to find your air/fuel mixture at idle to find out what the pair of carbs are actually doing.

 

[hangn0ut]

Street hemi carbs were staged with the rear carb primaries handling light throttle low rpm.

 

[Bigal75]

How do the plugs look? Heat range? Was the engine running well then started this?

 

[MrJLR]

Do a compression test.

Jeff

 

[Tempest 455]

I'd start with timing and vac test at idle.

 

[Jim Shook]

Then I'd go back to the carbs and their setup. Unless you have a completely divided intake manifold, one of the tricks we used to use, in days gone by, was remove one carb and use a block-off plate in it's place. Although your single carb may see small in CFM's it's more than adequate to maintain idle on a manifold with a common plenum. Set up the best mixture and smoothest idle you can get and then swap the other out and repeat. If yours is set up like a triple carb, only one carb handles the idle ( usually middle ) and the others acts as a secondary. Perhaps in your situation your running a bit rich and washing down the cylinder walls and plugs but you would know if you stood behind the car and inhaled or if you had an exhaust gas analyzer. I would assume there's something funky if you were to try and set idle with a pair of high CFM carbs because I would think there would be too much fuel atomization going into the intake or not enough vacuum for the pair to operate correctly. You really need to find your air/fuel mixture at idle to find out what the pair of carbs are actually doing.

Great suggestions! I did find that the front carb was leaking fuel into the carb throat. I
have a tech going back through them now to find out why. Hopefully that was my problem
Jim

 

[Jim Shook]

I'd start with timing and vac test at idle.

Thanks, tried that.

 

[Jim Shook]

Do a compression test.

Jeff

Yep! That's next on the list.

 

[Jim Shook]

How do the plugs look? Heat range? Was the engine running well then started this?

The plugs are new and the car had not run in 12 years.

 

[Jim Shook]

look to see if there's a piece of dirt in any of the idle jets or a clogged idle air bleed. a valve not seating fully can cause a rough idle but usually takes some rpm to smooth out.

Thanks
that seems like the best possibility, the front carb is dumping some gas on idle. So I'm going
back through the carbs now.

 

[oldbee]

Float level setting or a stuck needle in the seat..Recent post on here talked about a dog hair stuck in needle seat..Do you have a dog/dogs??lol.. Saw it a couple times; or one of your wife's hairs???? Don't laugh!!!!!!!!!!!!!!!

 

[lewtot184]

Thanks
that seems like the best possibility, the front carb is dumping some gas on idle. So I'm going
back through the carbs now.

if only one carb is bleeding fuel look for dirt and debris.