Looking for clues as to why my rebuilt 440 wants to diesel after the ignition switch is turned off. Carbs are new Year One purchases. Compression ratio is 10 to 1. Maybe a timing issue, vacuum issue, distributor issue, maybe the center carb is leaking gas into the intake? Anybody experienced this problem with their car? Thoughts?

Thanks,
Buddy

Center carb? so...a six pack?
Chances are that blades are open too far (in order to get it to idle usually) That is #1 cause
Six packs require a ton of initial timing to idle--if you don't have a distributor that allows this change --Mopar Performance that is no longer made with adjustable slots or an MSD then you are fighting an uphill battle.
Get 18-20 degrees of initial timing --that will allow you to close the throttle blades to correct adjustment and then you will be in the ball park

Do you have an idle stop solenoid installed and correctly adjusted?

I just went through this with my A 12. Car would not come down in idle and dieseled terribly.

What is current idle ? a high idle contributes to the dieseling issue. Once they solved the idle my diesel issue went away. I am running an idle solenoid as well.

Are your carbs reproductions ? My restoration shop and engine builder finally tracked the high idle issue down to the reproduction rear carb. They went through all the time honored search for a vacuum leak. Correct gaskets, throttle blades not shutting completely etc etc.

They did this by swapping a rear carb from a car that was running correctly and boom. My car ran perfectly idle speed was correct and no dieseling.

They then got Holley engineers involved and discovered a manufacturing defect in that lot number of rear carb. Holley sent a replacement. Same issue and same lot number. They eventually put a rear carb on from pro form or someone to solve it. The car is shipping to me now so I haven't got to hear it run but this has taken several months to chase down.

I hope your issue is simpler than what I went through. Feel free to PM me for more details if you would like.

Thanks everybody for your good thoughts. We now have some areas that we can focus on and perhaps pin point the problem.

Buddy

The new fuels are far more prone to dieseling in these old cars....In some cases, just putting some race fuel in will cure the problem if everything is in working order and it still diesels..

Agree on getting initial timing up...

MB