It took me a long time to solve this problem, and to be honest, the online forums were not much help! A friend of mine in Delaware R/C, who is very good with gas motors, immediately said it had to be a vacuum leak. It turned out that the rear crankshaft seal was bad. I now know a lot more about these motors, including how to tear them down and re-assemble, make gaskets, install seals, etc. He even helped me sandblast the cylinder, and the motor now looks and runs great, after 5 seasons of flying - pic attached.

Bottom line, if a 2-stroke won't shut down with the throttle closed, air is getting into the crankcase on the upstroke.

Mark