I had a bout with an engine which was run on older fuel. Eventually, I found that the piston sides were all varnished up. It seemed that this additional friction would cause the engine to drag over time while flying and eventually dead stick. Before we knew what the issue was, we found the engine to run quite warm, but could never fix the mixture to get rid of the seemingly lean situation. I pulled the engine apart, scrubbed the piston walls with solvent and a green scrub pad, until it was shiny new aluminum, then broke the engine in again as new. This is now a very sweet running engine.

I don't know how old your engine is, or if you knew the history, but I figured I'd share my experience.

Jack