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What causes an engine to run on after turning it off.

I built my stroker 360 about 9 yrs ago. I've had it run after turning the ignition switch off too many times. I've been told to decrease the timing, increase the octane....some days it does it(hot), some days not.

decreasing the timing is not the solution even when at times it indirectly takes care of the problem. Setting the timing back changes the idles speed and most time slows it. There for slowing idle and making the engine kill easier.
Tiiming the spark (other than changing the idle speed) cannot effect shut down. When you turn the ignition off there is no longer any timing. ignition spark is shut down. It is running on compression combustion.