Just in case that doesn't work.....

I was just remembering a strange problem I had with my brother's Ryobi some time ago. It had similar symptoms. It sounded exactly as if it were four stroking from a too rich mixture, and was nearly impossible to lean out. What I finally found was a bad reed valve seating area. The backplate had a crack in it, running near and through the reed valve area and the reed couldn't seat properly. I repaired this problem, and the engine ran perfectly.

AV8TOR