Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Dieseling

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Chevynation:
Two heavy springs worked. I’m still thinking about taking some idle bypass air out. With 18 initial plus 10 manifold vacuum advance it will idle no lower than 750 without touching the curb idle screw.

Kenth:
Set initial to 10-12° and use the ported source for vacuum advance.

If this don´t work, check the sizes of upper and lower idle circuit airbleeds, idle tubes size (at the lower end), downchannels size and idle mixture needle holes size.

Chevynation:
I’ll have to measure everything because it’s not happy at all at 10-12 degrees initial. It likes the 18 initial. Even with 18 initial and removing the vacuum advance I can’t notice an idle drop but looking at the timing it does retard so my pickup is indeed advancing when connected. I limited it to 10 degrees all in at 1.5 - 2” below idle vacuum. The ported vacuum advance port does not work with the carb since the main body was replaced. I never tried to make it work since I was only worried about manifold vacuum to my vacuum advance. I’m not a fan of ported to be honest. I was under the impression that I could simply remove some idle bypass air since it only affects how much the primary blades have to be open in order to get it to idle and nothing else. Essentially like closing the throttle blades.I would imagine doing so would require me to have to close the idle mixture screws in. The rest of the carb works fine. Responsive everywhere etc. Idle mixture screws are responsive as well. It can just idle with the throttle blades closed which is almost where I want the idle but not quite and I want the throttle blades to be open a small amount at idle i would think. I don’t want to recalibrate the carb and mess with everything that’s working fine. Just reducing the idle bypass air hole sizes in the baseplate by maybe .010 at a time won’t fix the issue?

Cliff Ruggles:
Just some info for those reading and following this thread.

ALL of the issues here are fundamental.  Too much camshaft and LSA too tight for the CID and compression ratio of the engine.

For sure "rowdy" tight LSA cams sound great but the overlap causes reversion, low vacuum at idle/poor signal to the carb and loss of throttle response and power at low RPM's. 

Doesn't mean we can't make things work but for sure the engine is NOT going to do well without running a LOT of initial timing and opening up the idle system to supply it with enough fuel to be happy at idle speed. 

It will also like a lot of idle bypass air to keep the throttle angle low so too much transfer slot isn't exposed and they don't "run-on" or "diesel when we shut they down........FWIW.....

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