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Intermittent rough idle.

bus driver

bus driver

Member
I have a 91EGC and my problem is an intermittent rough idle. When the engine is cold it is fine, but after riding and getting it up to operating tempurature, when I come to a stop it will drop from around 1100 to around 500rpm. I am about to do a winter project on it which includes a rebuild on the CV carb(rebuilt 10000) miles ago, a BUB exhaust that my buddy gave me( my Y pipe is cracked) and some other misc stuff. I just replaced the intake seals about 3000 miles ago, and I run Seafoam in it. It does not backfire on deceleration , and pulls very hard on acceleration.(as hard as a EGC can pull. It's like beating a school bus, what's the point). Otherwise it runs great. It only acts up warm and at idle. It will be idling at 1100, then drop to 500-600, then back up to 1100, then back to 500. When it's idling low, it feels like it's running rough. When you increase the throttle, it increases rpm with no hesitation. I just figured I'd ask around before I did the winter project, just so I don't overlook something while it's apart. It seems to be getting worse. I have to check the plugs. They are not that old, maybe 3500 miles. Sorry guys, I didn't check them before I wrote the thread , but I'll pull them when I get home. Thanx again for any advise.
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I haven't worked on a carb for some time now but a couple things come to mind. Have you checked for any air leaks? have you tried to adjust the air/ idle mixture?
 
Like Jeff I would suspect an air leak somewhere spray all the manifold / carb joints with wd 40 while the engine is at idle and see if the engine idle changes

Brian
 
fin_676 said:
Like Jeff I would suspect an air leak somewhere spray all the manifold / carb joints with wd 40 while the engine is at idle and see if the engine idle changes

Brian
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I will try spraying WD-40. Also I did adjust the air/ idle mixture. Like I said, there is no backfiring with deceleration. So that's why I wasn't leaning towards an air leak. But I've been wrong before, and I'm sure I'll be wrong again. You can ask my wife, all about that. Thanx guys.
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How does it run with about 1/2 choke when this happens?
 
Jack Klarich said:
How does it run with about 1/2 choke when this happens?
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Update. Well I pulled the plugs and it is definitely running lean.( white/grayish plugs). So while I was playing with it with the air cleaner and backing plate off, I gently moved the carb around, and it seemed to improve. I had an carb seal from a previous rebuild,so for the heck of it I put it on. It ran perfect for about 10 miles, then starting acting up again. There doesn't seem to be any movement with the intake seals. I had no luck with the WD-40 because it was constantly changing rpm. So it was hard to hear if itdefiantly it had an effect. I think for a cheap fix I might just try a new carb seal. While adjusting the air/idle mixture, it would improve, but then go back to a rough/500-rpm idle, then back up to 1100. I'm thinking new intake gaskets is in order. They are not that old, maybe 5000 miles. As soon as it cools off I'm going to pull the plugs again just to see. Thanx again guys
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Also, the rpm would increase slightly with the enrichener pulled half way. Almost like normal though. It was hard to really tell if it ran any smoother though. When I replaced the carb seal, that seemed to have the most effect. But I don't think that's the problem though. Thanx.
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If there is an air intake leak between the carb and the inlet valve then more air is drawn in which will make the mixture lean
Adding more fuel by pulling the choke knob to half way will make the mixture rich as it leaves the carb but due to air being drawn in after the carb a better mixture gets into the cylinder
I like to put a thin smear of silicone grease on the carb to manifold rubber seal to assist with installation removal and promotes a good seal

Brian
 
What did the plugs look like prior to having the current issue? I have to agree with Brian, sucking more air will surely lean it out. This may be a hit and miss fix but I'd be leaning towards an air leak.
 
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Pay attention to #6 if the carb does not fit snug here it will run poorly
 
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