I will certainly try a larger prop but I believe I've found a more pressing problem. I feel like an idiot. In taking it apart to check the valve clearance (WAY fat BTW) I took off the head and discovered a noticable lack of a head gasket. I mean, I'm kind of naive about 4-stroke engines, but ...

Moral of the story: never buy an engine on eBay - not even alleged "NIB." The money I saved is long gone in wasted time and frustration.

Unless TT .91 FS engines don't have head gaskets??