Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Poor Idle On Rebuilt Carb

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von:
The primary throttle blades must be installed one way only. Did you mark them before removal? Also the secondary blades have beveled edges and must be installed in the proper orientation too. If you installed everything exactly as it came out, it's always possible someone incorrectly installed them in the past.

Cliff Ruggles:
The secondary throttle plates need to be correctly indexed, centered and closing fully so you don't have any air leaks around them.  The factory used a pretty "loose" tolerance on the secondary throttle shafts so they can return to fully seated each time they close.  It's a pretty important part of the rebuilding process, so spend some time with the secondaries to insure that they are fully seated each time they close.

It's not uncommon to find a shaft that is twisted slightly, linkage holding them open slightly, or the plates not correctly centered up and indexed so they fully close.  If not done correctly, they are a vacuum leak and will not allow the engine to idle down low enough, and leans the idle mixture out at the same time.....Cliff

D Denzin:
How tight are the secondary blades supposed to be. They look good. But if you hold them up to the light there is a gap around them. There are touching on the higher side (higher up in carb). If I had to guess I'd say there is less than .003 gap. Any person with decent eyes can easily see a .001 gap with light behind it. You can see that they are narrower than the bore. So no way will they ever seal closed completely. Maybe I should take a feeler gage and check the clearance. I'm sure its very little.

D Denzin:
Ok, here is where I'm at.
I have fresh gas in the tank, I put in a new in line fuel filter.
I pulled off the carb and got out a feeler gage and checked the secondary bores. There is a small amount of movement left to right (side to side) of the butterfly shaft and butterflie assembly in the bores, its no more than .0015. No more than .0005 (that's right 1/2 of .001) of gap between the bores and the butterflies front to back when they are closed. I am sure that the fit of the secondaries to the bore is good.
I have to run the car with the PVC plugged.

At this point my idle is still poor. It isn't smooth, its a little rough, not real bad, but it has been better.
I can get it to idle at about 650. But what it does is it'll idle there for about 10 seconds then it drops down to about 500-550 for a second then it speeds back up to 650 and it'll idle there again for 10 seconds and it then repeats.

If I adjust the mixture screws out a half turn it'll loose the speed changing but then its idling at 700. I then back off the idle screw at the throttle linkage about 1/8 of a turn and then the idle drops but I'm back to the idle changing speeds. I started out with the mixture screws out 3 turns. I kept repeating the adjustments now I'm out 5 turns and nothing seems to be improving. Eventually I'm at the point again that the idle adjustment screw is not touching the throttle linkage. If I start cranking in the mixture screws then the idle gets real rough.

I don't see any dribbling of gas going on in the primary bores by the booster rings while its idling.


Any thoughts?
Thanks for all the help so far.

blazer74:
Must keep the PVC plugged.   Just a thought but are you using power brakes?

If so could you be loosing vac thru the brakes?

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