Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Idle air bypass?

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Cliff Ruggles:
That is a HUGE cam for a low compression 402.  The idle system would need significant modifications to have enough idle fuel, so most likely you are up on the main system and why there is no control with the mixture screws. 

Bypass air alone will not help that situation.....Cliff

72turbojet:
It really is too big of a cam. It was rebuilt by a previous owner and it's gutless down low with that setup. New heads are on the to-do list for sure.

But in the mean time would drilling holes in the primary blades help the situation? I'm starting to think I need to run a different carb until I get that compression ratio up where it should be

Cliff Ruggles:
I would NOT drill the primary throttle plates, the q-jet has an idle bypass air system which is more precise, and effective.

The engine needs more fuel to the mixtures screws, which means larger idle tubes, and idle down channel restrictions.  Once you get enough fuel to the mixtures screws for the low vacuum ready and rough idle cam being used, idle bypass air will help to get the throttle plates low enough to keep the main system off line (nozzle drip).

The best course of action here would be to replace the camshaft vs upping the compression ratio...IMHO.

I'd look at something around 214/224 on a 112 LSA, or even out on a 114.  Lots of the factory big block cams were really wide LSA, and they needed to be as they had pretty long seat timing.  Those cams made great power in big block engines........Cliff

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