DarZeelon and Pe,

Thank you for your inputs.

I don't know what the temps are but I've got a 36" shop fan blasting the test stand on high (directly in front of the engine, co-axial with the prop and flowing in the prop direction of flow). Ambient temps have been about 68 degrees F. I'm confident heating is not the issue here.

Again, an 18x12 prop is a bit much, but this engine should be putting out 3.8 bhp at WOT no matter what prop it's turning. Yes, the larger the prop, the slower 3.8 bhp can spin it.
What I'm trying to say is that my engine, regardless of the prop, is putting out nowhere near the advertized 3.8 bhp. The MVVS prop power calculator says I'm geting just over 1.0 bhp!

Does anyone know of any power-boosting tweaks I should have done that weren't in the manual? Any common power issues with these engines I could address? Thanks for your help.