Nope I just tried APC 16x4W. Engine unloads to 8100 rpm. That's it. Calculated HP it has much less than with bigger prop. I guess, I just hit the rpm limit set by porting timing or ignition timing. That puts APC 17x6 on the top of all APC prop for this engine. I mean for thrust, not speed.

However good news is that I was still able to start the engine by flipping the prop. Setup the same as the last time: glow plug powered from 1.2V NiMh, gasoline with Motul oil 1:32

RysiuM