rough idle only when engine is hot, can anyone help?
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 3,029
Likes: 94
From: meraux LA
rough idle only when engine is hot, can anyone help?
O.K. here the deal. my 350 runs great. It has a msd ignition and coil, fresh wire and plugs and the engine runs great. The problem is after it has run for some time it starts to have a rough idle. It doesn't kill but I can feel a miss and a slight stumble when i take off from a stop. The distributor is a new MSD set up for the vette's the with the 6a control box. It has no accessories and the vac for the headlights and windshield is disconnected. The carb is also pretty new maybe 1 year old. It is a holly 650 with vac secondaries. Any input would help. Thanks :confused:
Le Mans Master
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From: Semper ubi, sub ubi
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Re: rough idle only when engine is hot, can anyone help? (crf311)
I guess it could be caused by heat from the crossover in the intake. Maybe it is getting hot enough that the fuel is boiling in the carb. You could try blocking the crossover and putting an insulator (spacer) under the carb.
Race Director
Joined: Nov 2000
Posts: 11,872
Likes: 12
From: Waterloo ontario Canada
Re: rough idle only when engine is hot, can anyone help? (crf311)
If it runs good cold but not hot I would look for a rich condition. Your idle mixture could be too rich causing loading up at idle.
Le Mans Master
Joined: Jul 1999
Posts: 9,206
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From: Sewell NJ
Re: rough idle only when engine is hot, can anyone help? (crf311)
It sounds like you have a vacuum leak caused by expansion of the metal since it only happens when hot. You can check this out by spraying some WD 40 around the intake manifold and the base of the carb when the miss occurs. If you see or hear a change in the idle when you spray a spot you've found the leak. It could also be a mixture problem. When running cold the choke may make the mixture sufficiently rich to run well but when the choke is off the mixture may be too lean. Good luck with finding the problem.
Le Mans Master
Joined: Jan 2001
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Re: rough idle only when engine is hot, can anyone help? (crf311)
Try popping open your coolant system cap after it gets hot (carefully). I had a car that would do that after it got hot because I had head gasket leak INTO the cylinder. Once at idle the coolant system pressure would be enough to force water into the affected cylinder and cause it to miss. If I raised the engine speed then the cylinder pressure would keep the water out and it would run smoothly.
Drifting
Joined: Jan 2002
Posts: 1,645
Likes: 11
From: Schaumburg IL
Re: rough idle only when engine is hot, can anyone help? (Smokehouse69)
With a Holley you should not have the same problem that Q-Jets have with heat since the float bowls are not the same design. However, if you have an electric choke, try blocking off the exhaust crossover and do check the heat riser to make sure it is working. Or if you don't want to pull the manifold, try a nonconducting spacer and/or a heat shield for the carb.
Put a vacuum gauge on the manifold and see what you are pulling at idle when it is smooth before the total warmup and the miss develops. Then compare the vacuum reading when hot and missing. If it has dropped a couple of inches when hot and missing you have a leak when everything swells.
If this car has true duals, listne to the exhaust when running and missing - which side has a pop to the exhaust? At least thay may isolate which side is the problem.
EGR valve - have one? Pull the vacuum line and plug it. Better idle? Pull it and clean the ports or better yet, cut a thin piece of aluminum the gasket size and block it off, see if the idle smooths out. If so, if is leaking and needs a good cleaning.
That some ideas of starters.
Put a vacuum gauge on the manifold and see what you are pulling at idle when it is smooth before the total warmup and the miss develops. Then compare the vacuum reading when hot and missing. If it has dropped a couple of inches when hot and missing you have a leak when everything swells.
If this car has true duals, listne to the exhaust when running and missing - which side has a pop to the exhaust? At least thay may isolate which side is the problem.
EGR valve - have one? Pull the vacuum line and plug it. Better idle? Pull it and clean the ports or better yet, cut a thin piece of aluminum the gasket size and block it off, see if the idle smooths out. If so, if is leaking and needs a good cleaning.
That some ideas of starters.
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 3,029
Likes: 94
From: meraux LA
Re: rough idle only when engine is hot, can anyone help? (crf311)
Hey people, thanks for the input. You have given me some good areas to look into the problem. I will let you know what I find out. :cheers:
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Drifting
Joined: May 1999
Posts: 1,610
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From: Goulburn, Australia NSW
Re: rough idle only when engine is hot, can anyone help? (crf311)
I have the same problem with my 77, but I'm running a rebuilt Q-jet. The problem manifested itself before the rebuild tho. I suspect that I may be soaking some heat into the fuel through the stainless steel fuel lines I made up, tho the radiator/head leak is an interesting scenario. I'll have to try that one as it'd be a good excuse to fast track my new 383 build up.
