Thats a nice plane you have! I was running a 19x8 menz without issues in my now sadly demised cap232. You could try adding baffles to channel air thru' the cylinder head.Makes a huge difference as I see the crucial top part of the cylinder is not getting any direct air.The restrictive muffler and long extension could be another source.A shorter length could help or maybe remove the baffle.
Lastly could you be running the engine slightly lean? It becomes a problem as the fuel gets less in the tank and you're pulling into hover.I get that with non pump setups.