Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Poor Fuel Mileage on 4.3 V6 with Quadrajet 17056250

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don3194:
Well see, the sucking sound started while it was running. I had just got the engine warmed up after the rebuild and was about ready to start adjusting the throttle screw. Quite literally, I touched the tool to one mixture screw and I heard the sound start and the idle went up.

bry593:
If it wasn't doing it and then all of a sudden, with the engine idling, a sucking sound started and the rpms increased....  it sounds like a gasket was barely sealing, and then the idle vacuum was enough to unseal it.  Again, carb to intake gasket would be suspect as that is an easy one to hear.  Another possibility is the airhorn gasket since you can hear those leaks as well.  If your carb body is smashed down at the front two mounting bolt holes, then possibly the airhorn gasket is leaking slightly.  Obviously, both of these would make your idle lean, and would require additional turns out on the idle mix screws to add more fuel to make up for the extra air.  More air + more fuel = faster idle.

You'd be surprised how loud a vac leak in the carb is.  Simply opening idle air bypass (IABP) from .052 to .100 makes a very obvious sucking/whistling noise.

By the way, to tell if the airhorn gasket is leaking, you have to remove the airhorn and check the gasket for witness marks caused by the raised ridges on the top of the carb body.  The ridges trace around anything that must by 100% sealed.  So, if any part of the gasket is not showing these witness marks, that would be a suspect area.

Another thing that might make a sudden "sucking sound" is a ruptured diaphragm in your choke pull off.   Put a hose on the port, suck on it and block the end of the hose with your tongue.  If the vacuum bleeds off, then you have found your problem.

don3194:
Well I'm kinda feeling like an idiot, but I guess I'll just say make sure you use the right timing mark on your harmonic balancer.
As far as I could tell there was only one, but come to find out there are a couple on there. I'm not quite sure what the purpose of the one I was using is, but I retimed it to the right one and now I have the idle screw turned in about a 1/2-3/4 turn, there may still be a problem because the sucking is there and blocking the IABP didn't do anything.
Anyways it seems to drive well, the only thing is that if I go and floor it, it bogs down quite a bit at lower rpms once the air doors open.

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