Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Quadrajet Sputter Turning Off

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Chevynation:
Hello, my 7040203 quadrajet is causing a sputter when turning the engine off sometimes. I have the throttle blades practically closed and the idle at 800. My carb does have idle bypass air so this allows my throttle blades to be closed more and I bumped my initial timing up and used manifold vacuum advance. Is this normal for a quadrajet because my Holley doesn’t have this problem.

Cliff Ruggles:
800 RPM is pretty high for curb idle speed.  Is it "running on" when you shut it off or spitting up thru the carb?.....

Chevynation:
It’s more of a run on situation. It will run on for a very brief moment instead of just stopping when I turn the key. I lowered the idle to 700 but it’s a little low because when I turn my heat on full blast it will drop the idle too low so I have complete control of the curb idle. Of course when I back the initial timing down it’ll be worse because I have to open the throttle blades more but I barely have the curb idle opened so I don’t have too much transfer slot exposed.

Chevynation:
Maybe an idle stop solenoid would help with the rpm drop when I turn my heat on and I can lower the idle.

Cliff Ruggles:
Run-on can often be caused simply by the idle speed too high.

Octane is another culprit.  If you have really "low" octane fuel and decent compression you can get some "run-on" at shut down when the engine is fully warmed up and well heat soaked. 

I used to see this once in a while with the 455 in my Ventura.  It only did it on really hot days and well heat soaked with lower octane fuel.  Never had it run-on with 92-93 octane in it but anything less it would do it on occasion......

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