Quadrajet seems rich on idle?? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 09-10-2011, 02:16 AM
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Quadrajet seems rich on idle??

I have an 86 GMC 1/2 ton pickup with a Quadrajet that came off an early 80's cadillac according to the numbers, the engine is a pretty fresh rebuilt 305 with a Comp XE 274 knockoff, long-tube headers and dual 2.5" exhaust with a Pypes X-change kit and flowmaster 10 series, my problem is that since I've had the truck, only about a year now and before the stuff above was added, it had an issue with smoking at idle, it smells like straight gas and wasn't too bad at first but in the last week it's gotten pretty rough, it runs great though, no bog, likes the cam, good idle, just the smoke, would a simple rebuild fix this? Im assuming that's it because some gaskets look like they could be leaking. I know, I can do some engine work, I outta be able to figure this out but, this is my first carb'ed engine and I know the reputation of the quadrajet. Sorry for the newb stuff. Thanks for any help. Just wanted to know if it'd be as easy as I'm thinking.

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Last edited by 94grand5.2; 09-10-2011 at 02:25 AM.
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Old 09-10-2011, 04:18 AM
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As you have already stated a carb of questionable or unknown condition could benefit from a simple rebuild. Pay particular attention to cleanliness and getting the float height correct. I agree that leaky gaskets are bad.

Interestingly, a carb that is actually lean in the idle circuit can display symptoms of richness at idle. This results from a carb that stubbornly wishes to run without the benefit of sufficient fuel from the idle circuit. How does it do it? It tricks its owner into opening the throttle blades too much to keep it running exposing too much transfer slot or worse yet inducing the dreaded nozzle drip (fuel being pulled from the main boosters at idle). Insufficient idle bypass air can also induce this condition. I guess my point if I have one, is to make sure the throttle blade angle is set in the correct relationship to the tslot and then if it won't idle properly find out why.

Stick your nose down the carb will idling (carefully) and see if you see fuel dribbling out of the mains. If no drip try putting a kit in it and see if that helps.

Correct ignition timing is an important part of the equation.

There will be other more knowledgeable members along shortly to help. This site likes a good Qjet mystery.

Last edited by TommyK; 09-10-2011 at 04:25 AM.
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Old 09-10-2011, 07:28 AM
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Okay, first off, where is the spark timing set? Second is vacuum advance in use? You may not think this has anything to do with it running rich at idle but it does. Third, Does the carb have the computer hook ups on it?
Fourth you have way too much cam and not enough engine. That cam makes a 350 which is 45 cubes bigger a rough customer making it way too big for a 305.
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Old 09-10-2011, 07:00 PM
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Well it seems like I might have gotten a start on it, one of the idle screws was backed out a little further than the other, at least Im assuming thats what it was, just gave it a few turns to even em up. Still a little smoke but not the huge cloud I had before. And the throttle shaft has all kinds of play in it, I've seen that's a common problem too. This all from what I've read, never actually looked at any of this until now. As far as timing, we do it the fun way with no light and I was lucky enough to get the engine with no timing tab, just do it all by ear the best I can and yes it has a vacuum advance dist. And I know that cam is probably more than I should have ever bought but, it hasn't been in there any time at all and I have a 350 sitting I could always start on.
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