[nutz4sand]

If a motor runs on after you kill the ignition its usually called "dieseling" cause its running with out a spark plug so to speak.

This is usually caused by a VERY hot motor and the tip of the spark plug or a bit of carbon in the cylinder is soooooooooo hot its igniting the gas mixture and making the "spark" this way.

Its not something you want to be happening. It is many times a sign somethings not quite right as its not supposed to happen.

I see your in Nevada but bau if you machines jetted for 90 plus degrees and its reallly colder now the jetting may be lean and making it run hot.

The carbon may be from the former owner never running hard IF thats the case.

When you shut it off did it run really smooth still or sputter a bit and miss any till you gassed it to kill it?