Is your tank mounted on the CG- or behind the firewall? Maybe the engine is having a hard time pulling fuel from a CG mounted tank set-up?

You stated that you are not the original owner of this engine.

Maybe it was not broken in properly.

If it got hot during break-in; then it might be ruined. I've seen a few guys do it. I ruined an OS 120 Surpass this way. After an engine gets too hot during break-in; it will never run right. It will heat up after a few minutes and start to sag and loose RPM's. Then it quits.

Maybe a new piston, ring, and cylinder will bring new life to it; if all else fails.

Just a few ideas for the stew.