Oops...I missed that you already tried the throttle trim....sorry about that.

To add to AuthorJim's suggesations, your fuel mixture could be screwed up by a fuel tank pressure leak. Maybe the fuel cap o-ring isn't sealing correctly, or the pressure line has a split or worn through, or the fuel tank has a crack in it,.... perhaps even the exhaust system has come loose or has a leak somewhere.

There's a lot of things to visually check. Loose carb, air-leak at pinch-bolt, loose cyl head, etc. The clutch could even be sticking.