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.