'86 silverado dieseling down?

Question

I have an '86 silverado. I put a new 350 in it. New carb and new almost everything. When I shut the key off, it diesels. I have had so many people look at this problem. I even have the idol turned way down to the point where I am stalling at lights. the timing is right on the dot! No one can tell me whats going on! the way I have been shutting it off is just flooding it after I turn the key. I've shut it off in drive, and it still diesels down. Sometimes when its warm out, it will shut off perfect, other times it won't. I have even had the vacuum advanced replaced and the distributor cap replaced. The choke is also set to where it is supposed to be. I need answers!!! PLEASE HELP!! -Thank you

Answer 1

There is supposed to be a solenoid bolted to the base of the carb that pushes the throttle open slightly when the ignition is turned on.It allows the throttle plate to shut completely when the ignition is turned off.They were installed as standard equipment on millions of GM vehicles before fuel injection.

Answer 2

two possibilities, 1) carbon deposits in the engine elevating the compression ratio,and or creating a hot spot to ignite incoming fuel. easy fix, steam clean it, ( iv done this to my Honda.) disconnect cat, run engine with a steaming kettle of water pointed at the intake allowing steam to enter the engine. run for 15 minutes. just for kicks, measure your compression pressure before and after. mine dropped 35psi in 2 cylinders. reconnect cat and test drive.
2) air fuel ratio is a little off (lean) locate idle mixture screws and turn out 1/2 turn and test drive.