It's premixed new gas and I did measure before I bought piston?
oil injection or are you pre mixing?,stale gas
airscrew setting?
compression test?
baffle in that stock exhaust header could be crapping the bed
who knows....if you did not measure bore/honed/bored before you bought piston,have not leak tested,have not compression tested....who knows, it was probably overheating if your gaskets were bad and it was being driven.