Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Dieseling

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Chevynation:
Hello, I have a quadrajet that runs on when I have the idle set at 800 which is where the engine likes to idle. If I lower the idle to close the throttle blades then it will not run on but the idle will be too low at that point. I increased my initial timing but it’s still not enough. No vacuum leaks and I even tried using manifold vacuum on the vacuum advance. Idle mixture screws have full affect and I can’t see any dripping from the boosters. I read about idle bypass air on my carb model but how can I increase it so I can close my throttle blades to prevent run on at shutdown? I have a manual transmission as well.

Chevynation:
Correction, I know I can increase them by opening the size of the holes on the base plate. Depending on the size of the holes how much do I need to increase by in order to see a difference? I shouldn’t need to increase idle fuel correct? The carb was rebuilt and runs great all around. I’m just trying to slightly tweak it to correct this issue run on issue that’s all. I know another option is an anti dieseling solenoid but I prefer not to run it.

Chevynation:

--- Quote from: Chevynation on October 14, 2023, 08:41:30 AM ---Correction, I know I can increase them by opening the size of the holes on the base plate. Depending on the size of the holes how much do I need to increase by in order to see a difference? I shouldn’t need to increase idle fuel correct? Cliff rebuilt the carb years ago and it runs great all around. I’m just trying to slightly tweak it to correct this run on issue that’s all. I know another option is an anti dieseling solenoid but I prefer not to run it.

--- End quote ---

77cruiser:
What are the specs of the engine? How much timing are you running at idle with vac. advance?

Chevynation:
383 288HR cam. Duration @ 0.006":288° / 294°Duration @ 0.050":236° / 242°
Max Lift w/ 1.5RR: .520" / .540" Lobe Separation:110°. I have 1.6 rockers. Dual plane edelbrock performer rpm manifold. AFR 195 heads 10.5:1. Had 18 initial and 28 with advance plugged in. I limited the vacuum advance to 10 degrees. I currently have 22 initial and the total is 34 degrees which the engine likes best with the efficient chamber heads.

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