Well, when the idle timing is not advanced far enough with a healthy cam, the blades, thats butterflies, or throttle blades, have to be opened quite far to keep the engine running. Then at engine shut-down the engine is able to draw in air past them, with accompanying fuel. Then if there is sufficient heat in the chamber, the mixture will attempt to burn, and you get run-on,also called dieseling.
The cure is to give the engine the timing it so desperately craves. Then the blade-opening can be reduced. The result is a smother idling engine with a better tip-in response, and no more dieseling.
When this is done, most times the maximum power timing is also increased perhaps to the point of detonating under vfull-load/fullRPM.This has to be avoided, or the engine will begin to self-destruct