Quadrajet idle problems

[airboatgreg]

I have built hundreds of QJ's. Awesome carb and if it runs good don't ever get rid of it. We can re-bush with valve guide buishings to take the throttle plate play out. The newest ones are 20+ years old with CCC. Non CCC 30+. The Carter made QJ's are junk as they disintegrate. One issue we had with high idle were the secondary's plates were allowing air flow creating a higher then desired idle. We had a coating we would put on them but that has long dried up. Sometime you can LIGHTLY tap each end of the throttle plate to re-center the plates in the bore. See if you now have vacuum to the distributer advance at idle now where maybe you had ported before.

 

[Bludacious]

Sometime you can LIGHTLY tap each end of the throttle plate to re-center the plates in the bore.

Sorry for the late reply, turns out Greg was pretty much right on the money! I realized part of the problem was until now I adjusted the mixture screws with the idea that they were supposed to take the main bore totally out of the idle circuit, so I had the curb idle screw backed out to close the blades basically 100% at idle. Don't ask me where that info got stuck in my head, but I readjusted the curb and mixture screws and discovered the real problem. Surprise surprise, the main throttle plates are either slightly oversize or not centered because the throttle sticks at the very bottom of its motion range, and now that it's colder out it very obviously binds until the car is good and heated up. The blades at any temp stick in just such a way that they just barely are open enough to be up off the curb screw and raise the idle about 100-150 in or out of gear, which is why I could never adjust it reliably. If I adjust all screws properly and gently push it down after goosing it with my hand it idles at a cool 800/600. Either way now that my curb screw is properly cracking the blades I can reliably cut it in park below 65-70F without the motor complaining. The shafts themselves have a bit of wiggle but no worse than the other quad I ran on it in the past year which idled at the right speed and without any major shuddering / mixture issues. Is there a procedure for properly re-centering or shaving down the blades? I would need to remove the base plate from the carb to do that as well I presume. In the meantime I will try the tapping trick and see if I notice any improvement.

 

[Clone TIE Pilot]

The plate screws are usually steaked, so to completely remove the screws you need to grind the steaked sections off to avoid breakage.

 

[69hurstolds]

It's totally an awesome feeling to have a rebuilt carburetor that you have to work on to get right.

Next time you need a Q-jet rebuilt and don't want to tackle it yourself, I suggest you go with Everyday Performance. Who just happens to be a member here. http://www.everyday-performance.com/

 

[hangar401]

I too had fast idle trouble with the original Quadrajet on my 305.Fast idle screw can be found below the vacuum pot on pass.side of the carb.Have to loosen off the pot and screw is visible, solved the problem.

 

[Bludacious]

So if I'm understanding this correctly the proper way to center the blades is to just loosen the screws (the rebuilt one has loc-tite'd screws with no peening), tap them lightly until they seat reasonably well, then re-tighten the screws? If the blades are centered right and are just oversize should I sand/shave down the blades or the plate? I'd lean more towards the blades because they can always be replaced. I also have another base plate on the last Qjet I ran that's a '76 that looks like it was new when it was remanned in 2002-ish and has very little play in the shaft. Would it be interchangeable with the 80-86 main body on the new unit? I can get the casting #'s off both if necessary.

I too had fast idle trouble with the original Quadrajet on my 305.Fast idle screw can be found below the vacuum pot on pass.side of the carb.Have to loosen off the pot and screw is visible, solved the problem.

I've always ran with my fast idle screw backed out all the way. Wears out the motor quicker and wastes an ungodly amount of gas when I let it warm up in the NH winters. I think my record was 1/6 of a tank from 10-15 mins on the fast idle cam set at stock speed.

 

[L92 OLDS]

Or just buy the good used 800 cfm Q jet I have for sale. Check the classified ads

 

[Clone TIE Pilot]

So if I'm understanding this correctly the proper way to center the blades is to just loosen the screws (the rebuilt one has loc-tite'd screws with no peening), tap them lightly until they seat reasonably well, then re-tighten the screws? If the blades are centered right and are just oversize should I sand/shave down the blades or the plate? I'd lean more towards the blades because they can always be replaced. I also have another base plate on the last Qjet I ran that's a '76 that looks like it was new when it was remanned in 2002-ish and has very little play in the shaft. Would it be interchangeable with the 80-86 main body on the new unit? I can get the casting #'s off both if necessary.

I've always ran with my fast idle screw backed out all the way. Wears out the motor quicker and wastes an ungodly amount of gas when I let it warm up in the NH winters. I think my record was 1/6 of a tank from 10-15 mins on the fast idle cam set at stock speed.

I don't know about swapping base plates. Qjets are very application specific carbs, with each have subtle differences for each engine option application. This includes differences in the castings, how the passageways are drilled, sizing, etc.