I have a 75 f250 with a 390 and when i shut it off the engine will idle real low for about as long as it wants. I usually pull the choke and that kills it right away. i just set the timing and the idle is where it should be so if you all could help that would be great.

 

That goes back a bit, but some carburetors had an idle stop solenoid that as long as the key was on the solenoid held the throttle open to the correct adjusted idle speed then when the key was turned off the solenoid would retract and allow the throttle butterflys to close thereby dropping the idle speed so low the engine would shut down promptly.

They got the name of an anti-diesling solenoid.

 

I know if the pistons had a lot of carbon on top, it would be hot enough to ignite the gas and you could get run-on for a few seconds.