Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Dieseling

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Mudsport96:
No expert but 22 initial is a whole lot of base timing. My inefficient heads don't need that much. Start by backing the timing down around 14-16 initial with no vacuum. And try resetting idle screws.

Chevynation:
Since I don’t have enough idle bypass air when I decrease my initial I have to open the curb idle screw too far and expose too much transfer slot so it starts dieseling. The cam really likes 18 initial but I just bumped it up to 22 for a test. I took the carb off to see how much transfer slot is exposed and it’s too much.

77cruiser:
Does the idle speed change much when you increase the timing from 18 - 22 - 26?

Chevynation:
Absolutely. At 18 I could only get it to not run on at about 600 rpm or so and at 22 I can get it to idle at 750 without running on. 750 isn’t bad but it’s a little low at times especially with heat on. It likes 800 but it will run on. So I just need a small amount of additional air so I don’t have to have the idle screw open as far. I also want to see why my full manifold vacuum port on the side of the carb doesn’t have any vacuum. Neither of my ports have vacuum but the carb has a different main body than it originally had. Hopefully it’s just the gasket that needs a hole drilled but it shouldn’t be too difficult to fix. I’m not worried about the ported vacuum port so that will just remain inoperable. Will .010 increased idle bypass size be sufficient in your opinion? Depending on how the size of hole in the baseplate is I’ll either install a brass set screw so I can drill to the size I want but if the hole is small and just has epoxy then I’ll just drill it a little larger.

77cruiser:
Have you tried more timing just to see what happens?
What size is your IAB now? Guessing that can you probably have 10-11 in.  of vac at idle.

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