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TNshadetree 07-16-2007 08:12 AM

Quadrajet Idle Strangeness
I've installed a rebuilt quadrajet and a Performer intake on my pretty much stock 305 daily driver. The rebuild of the carb was very detailed, even removing and cleaning out the idle tubes and blowing out all passages. The throttle shafts have bronze inserts already installed and I've checked and re-checked the float level, but I still have a strange idle condition.

When the car is warmed up it will idle at a consistant rpm then drop and idle much slower for 5 seconds or so, after which it goes back to the faster idle which it will maintain for about 10 seconds before dropping down for another 5 seconds, etc. etc.

I've attempted to look into the carb while it does this, but I've seen no evidence that the main jets are expressing gas when this happens. This carb has the adjustable part throttle, but adjustments to it have had no effect on this condition, nor has any of the adjustemnts to the idle screws. I now drive the car idled higher than I'd like just to keep it from stalling when it does decided to drop down to it's lower idle speed. I've though maybe the fual bowl is being overfilled by a faulty needle seat or something. But it doesn't figure that it would overcome that within 5 seconds of only idle consumption.

Any ideas? I'm a quadrajet fan and I've owned some great ones, but this has me miffed. I've been trying everything I can think of and nothing seems to help.

302 Z28 07-16-2007 08:37 AM

Assuming the idle passages are all free of obstructions, I would suspect a vacuum leak or fuel puddling on top of the butterflies. Do you have good adjustability with your idle adjustment screws?


DHMag 07-16-2007 03:52 PM

im thinking its something outside the carb. EGR, PCV...

DoubleVision 07-16-2007 04:15 PM

If the idle screws have no effect there has to be another reason for it just as Vince said. A vaccum leak or fuel puddling. Years ago I had one that had to much fuel pressure, and it was flooding out of the idle passages so you couldn`t see it, and it was very difficult to figure out, but what gave it away was, after I would shut the car off it would continue to smoke out of the carb for about the next 10 minutes, that white hazy looking mist would rise out of the carb for a long period, that`s what told me something was amiss.
What is the fuel pressure? have you made any changes to the fuel system? is the vapour line still open that goes to the fuel tank? is the carbon canister still in use?

F&J 07-16-2007 05:58 PM

things on quadrajets I've seen:

-leaking welch plugs on the bottom of the main body casting. These are notorious for leaking, and most rebuilders put a special sealer over the plugs when the base is off. This also causes the bowls to be empty in a day or so, if you don't drive it much.

-Internal vacuum leak caused by warping. This usually makes the car almost seem like it's got a slight "cam", by the way it idles, and adjusting the mixture does still afffect idle quality, but can't seem to get it stable. Just went through this with a late 70s vette. It had a new kit and new gaskets did not help.

31choptop 07-16-2007 07:36 PM

I agree the plugs can and do leak. they need to be checked and or epoxied

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