General Category > Quadrajet Carb Talk and Tips
What are these holes for?
Cadman-iac:
:-*On the primary air bleeds in the air horn, do you go with the 10-32, or a 1/4-20 set screw?
Just practicing on a core, i noticed that you can use either one, it just depends on how big you want the drilled holes in the set screws.
But it's easy to drill a small hole in a big screw, much harder to drill a big hole in a little screw, your hex drive disappears.
77cruiser:
Only time I ever tried different MAB sizes was one a carb removed the outer booster to try it & I mess with the bleeds but never got it to work. But I think I used 8-32 I think the h.ex is 5/64.
Cadman-iac:
--- Quote from: 77cruiser on November 06, 2025, 06:24:50 PM ---Only time I ever tried different MAB sizes was one a carb removed the outer booster to try it & I mess with the bleeds but never got it to work. But I think I used 8-32 I think the h.ex is 5/64.
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Thanks Cruiser, I put some 8-32 screws in it. Those are big enough for up to an .080" hole in think.
Rick
novadude:
--- Quote from: Cadman-iac on November 06, 2025, 03:26:23 PM --- On this subject Novadude, if you increase the UIAB's this will lean out your idle mixture, if im understanding it correctly.
--- End quote ---
Yes. I believe that is true.
--- Quote ---By increasing the LIAB's then this will fatten up, "enrich" the off-idle mixture, again if I'm understanding it correctly.
--- End quote ---
I'm not sure about this one. I think larger LIABs will actually lean it out until the throttle blades get past the LIAB. By this point, the main system will already be active. In my experience, the larger 0.078" LIAB carbs never seemed to run as well off-idle as the ~0.062 LIAB carbs.
--- Quote --- Enlarging the idle tubes enriches the whole range of the idle circuit, correct?
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I think this is true
--- Quote ---And by increasing the IDCR's, this would have the same effect as increasing the idle tubes, but also increases the effective range of the idle circuit, is that correct?
If not, what am I missing here?
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Larger IDCR will increase the amount of mixture getting to the transfer slot and idle screws. If the UIAB is too large, or idle tube is too small, it could still end up lean off idle. At least I think that is how it works.
--- Quote ---And increasing the idle by-pass air it allows you to close the throttle plate more to prevent any nozzle drip, in addition to being able to increase the idle speed without changing the throttle position, I think, right?[/quote']
Agree
I was up late last night reading Doug Roe's books trying to get it straight in my mind.
Another question, going through what I need for various sized drilled set screws I couldn't remember where the 10-32 screws were used, or did I order those by mistake? Do you use this size for anything?
Kenth, Novadude, thanks for all the help and patience on this. Im beginning to think that I don't understand everything I know about this carburetor, lol!
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77cruiser:
--- Quote from: 77cruiser on November 06, 2025, 05:15:24 PM ---I used the 10-32 to make idle tubes, you need some extra tubes. If you have a couple of junk cores to pull the tubes out of you're set. I can't remember off hand the length of the screws 1/2 in. I think & the tube have to be shortened the depth of the hex in the screw.
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OK I see I put 1/2 inch should have been 1/4 inch. I'll measure later to make sure.
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