Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Poor Idle On Rebuilt Carb

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68 Firebird:
Most carb issues are actually ignition issues!!!  That is a true statement I learned a few years ago.
When I had my DUI HEI curved I had them setup curve to start at 900-950rpms.  This worked wonders
for my idle in and out of gear.  And running ported vacuum advance takes another variable away from
idle equation.

D Denzin:
Well I pulled the dist apart and found it had apparently the wrong weights in it. I checked them against an old dist I had laying around. So I swapped them out. Car ran better. I ended up reattaching the PCV and swapped vac advance over to the ported source. Car ran OK. I turned car on and off. Everything seemed better. No changes.

I figured I check total timing, adjust mixture screws using the vac gage and do any final adjustments.
I thought I had it licked.
While I was getting my stuff ready to restart and do the adjustments I notice there is smoke coming off engine on the pass. back corner. Where the PCV goes into the Valley cover. I figured something was screwy with the PCV. Well long story short it seems the coil was leaking oil. I could see oil bubbling out of the rolled seam on the top of the coil. This was causing the smoke. I pulled the coil and appparently the black plastic top section is bulged and cracked. Its also low on oil. You can hear the oil sloshing around when you tilt the coil over.

So now I need to get a new coil. I was using a Pertronix coil to go with the Pertronix ignition set. I'm going to get my hands on a old GM coil and try that. I might be able to prevent a new pertronix coil from going bad if I remount it up and away from the engine but it seems that the original GM stuff lasted in the original location why shouldn't the aftermarket one? Off to find a coil and try again.

Thanks again all for helps and tips. I'm optimistic that I'll have this licked soon.

Cliff Ruggles:
Progress is a good thing.  The distributor is a big player with these things, and you've got to get it working correctly before carb tuning will be effective.

You didn't mention if the timing was actually moving around at idle speed or not?

I would also check the mixture screws now that you have went to a ported source, to make sure they have enough fuel for the engine without backing them out too far?.....Cliff

D Denzin:
Timing is solid. doesn't move around at idle when using ported or plugged. Moves slightly when idle is fluctuating when I'm using manifold vac source. Centrifugal advance does not kick in till around 1200 rpms.

Mixture screws were turned in about 2 turns on ported vac. Seemed to help smooth out engine. I was going to do some final adjustments as mentioned in last post when I noted bad coil. So thats where I'm at now.
Thanks for all the help. I'll advise what happens when I get a new/different coil.

Always seems to be 2 steps forward one back.

D Denzin:
So I got a new coil.
I can get it to idle at a decent rpm. 600-650. That's good.
It doesn't idle smoothly, kinda labored. I get about 13-14 in. of vac at idle.
I've got timing at 13 deg at idle. Ends up giving me about 35 total with no vac advance. Am using ported vac source for dist.
I've got the PCV hooked up.

 I've got the mixture screws out 7 turns. Seems best about there. Not a lot of difference in the adjustments of the mixture screws. If I get them in too far, maybe 2 turns or less it really gets rough. I can crank it out and all it does is increase the idle speed alittle. Does not really make the idle smoother. So then I adjust the throttle linkage to lower the rpms. Shouldn't I see more of a dramatic difference in adjustments on the mixture screws? How far out is too much? how do I tell if its too rich on the mixture screws? I'm hesitant to go beyond 7 turns out. There is no resistance from the springs at this point and the mixture screws seem very loose.


So all in all its idling at a decent speed but its idle is not smooth, minor surges and it floats up and down between 600-650. The Vac is lower than I think it should be and it too floats between 13 and 14 in. I checked the timing with the vac advance disconnected and plugged. Timing is solid. Removing the vac advance does not change the timing as I would expect it shouldn't when using the ported vac source. When adjusting timing I notice that the engine speeds up and smooths out when advancing the timing to about 20-25. Why not go back to man. vac for the dist advance? If there is no downside that anyone can clue me into I'm going to try that.

I got a couple of other issues fixed while waiting for the coil so I could take it for a spin down the road. Seems to run pretty good.  No bucking, secondaries open nicely. I would say that other than the idle this carb seems like an improvement from the stock AFB that was on the engine.

On last issue with the idle. When I turn the car off and restart it after any period of time the idle decreases about 100 rpms. So after restart its idling around 550. running rougher too. A couple of blips on the throttle doesn't cure it. But it seems after about 3-4 minutes of running it goes away. The car is also running hotter than "normal" when idling for about 15 minutes or so. So when I took it out it was on the hot side. Only took about of minute of driving and the temp was down to more of a normal range. Maybe some sort of heat soak thing going on with my idle issues?

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