[Excaliber]

You know if the motor was still running because the MSD is still getting electric power then it would seem the motor would run/idle fine. Nice and smooth, timed correctly, electric spark from the spark plug igniting the fuel/air mixture at the proper time. It just won't shut off, yeah you could call that "run on", but the term may be incorrectly used in that manner.

"Run on" or "dieseling" most generally refers to a condition where the spark plug is NOT firing, the motor is STILL running but very rough, likely it is backfiring through the carb, it may even be running backward at some point just before it dies and in general sounds terrible!

The MSD problem is troubling and a hassle but it won't damage your motor, whereas "run on" or "dieseling" could seriously damage your motor.

So, is this "run on" condition of which we speak "smooth" or "rough" in nature? To entirely different conditions. IF the MSD does shut down within a few seconds after you turn off the key you might THEN have a "run on" condition that is seriously rough in nature. Fixing the MSD won't make the second part go away.