What usually happened is after a few years, which back then is like ten years of wear and tear now, carbon builds up on the pistons and increases the compression and also absorbs and holds gas, so when you shut down the motor the engine "dieseled" due to the higher compression (on an already high compression engine) and the gas being present in the carbon build up for your fire. Before the first rebuild of my engine, I remember being able to shut it down and while it was dieseling, just turn the key to run and it would continue on not missing a beat.