Todjob, If the dieseling you are trying to eliminate is "running on" after the key is shut off then the answer usually is like 3b said--the throttle opening at idle is too great or possibly a vacuum leak.
Set the idle speed down to a minumum and see if that doesn't cure it.
I don't see how timing setting or advance mechanism can affect running on since the spark is stopped when the switch is cut off so distributor timed spark is no longer a factor.
Hot spots in the comb chamber provide ignition after the spark is stopped but closing the throttle to provide minimum air flow therefore lowered compression will shut her down.
In the early emission lowering days the factory provided a way on many engines by solenoid activated throttle stop to allow the throttle plates to close tightly when the switch was cut off to minimize running on (dieseling)but when the solenoid was energized it extended to allow a normal idle speed.Some non automotive engines that usually are not idled down before shutdown have a way of blocking by solenoid the main jet when the key is shut off to prevent running on and backfiring.Look at your larger lawn mower engines like B&S and Kohler.
My guess is abnormally high idle speed.
