Since it began life in another machine, and then has been flown too for god knows how long, I would suspect a worn ring. Most every used engine I tear apart has a ring gap that is too wide/out of spec. The ring gap spec for this size engine is .000" to .003". About the thickness of a piece of paper. Put an $11.00 Frank Bowman ring in it and I think you'll see a big difference. I put them in every engine I own. I have had engines that ran really good to begin with gain 800 rpms with a new ring.

The Zoar props do load an engine down more than most others.

AV8TOR