Runaway engine....
I've seen this a few times in the last couple months. It's noted in the manuals as a runaway engine. I'm not 100% sure what causes it because I haven't done a lot of research on it. I do know that manual says not to rev them a lot without torque on the impeller shaft, to keep a load on the engine.
The way I remember reading it in the manual (about a year ago), was that when you speed the engine up, it goes into free wheeling and the inertia conitnues to speed up the engine, heating and burning a plug, then when you cut the power, the heat of the engine will continue to combust the fuel in the cylinders. The only way to shut it down is to use the choke.
I'll be looking into this more and hope to post this in my "korner".:cheers: