D17 IV Engine Run-on After Shutoff?

This is not a new problem, and has happened occasionally for as long as we have owned the tractor (30+ years?).  The engine is in excellent condition and runs well under all conditions.  The tractor is used only for light work.  Once in a while, and it is hard to predict when, the engine will run-on or run backwards for a bit after shutting the key off.  As a habit I always let it idle for a minute or so before shutdown, even if it has not worked hard or is not up to temp.  It only happens once in a while, and I don't see any pattern to when it might happen.  I put electronic ignition in a few years ago, but it did the same with points. It gets very few hours a year, and the plugs and oil are usually changed out long before they are even dirty.   Is it carburation or electrical?  How bad is it for the engine when it does this?  It typically only does this for maybe 5-10 seconds before finally shutting off.  As I said, it is not necessarily related to its being hot or after a long work session, although I always idle it long enough to cool down if it has been worked.  Carbon buildup in the combustion chamber?  I am curious as to what might cause this.

points and plugs so it is a gas engine... Gas engine normally will not RUN with out a spark.. NORMALLY the compression is not enough to get ignition.. Carbon buildup in the top can be a cause..... another Possibility is the ignition switch is worn out and when you turn the key off, you still have an electrical path to the coil to give spark... and the vibration eventually causes the switch contacts to fall off.... I would try a KILL switch to the coil as a TEST to see if the switch is the cause. 

Agree idle has to be SLOW! Wife doesn't quite close throttle on 45 sometimes and it wants to run on a bit and the knock is frightening. My mechanic dad said that's hard on brgs rings and crank.

Edited by SteveM C/IL - 5 hours 47 minutes ago at 10:12am