Gyri:

Wow, never saw or experienced anything like that. At this point I suspect your fuel at 10%, should be 20-20 for the YS. Has the engine been backfiring or other indications of detonation? Frankly I would send the engine into a YS service center to be rebuilt. My experience has been that the centers do a outstanding job at a very reasonable price. Am sure other help will chime in.