BBri Bro-- just saw this post on your Honda 4-stroke application. What a gorgeous plane! I have been looking at these engines for a while but until now had not really heard how well they worked in a firsthand report. I had e-mailed Dave Carr a while back regarding mounting these engines inverted because of my concern of crankcase oil seepage past the rings in between flying days, as well as how they ran inverted (although I wasn't too worried in that respect due to weedeaters being used that way all the time). His reply was that they would work fine while mounted inverted, but should be stored upright for that reason (seepage past the rings). The fact that he recommends inverted mounting and one of the other posts saying his ran fine inverted suggests an isolated problem with your setup. I don't have any input or suggestions, just an observation. One other observation that I'm sure you're on top of: have you tried a dual pitch prop, like a 20-6/10? There are a couple guys at our field using gas engines in their WWI and WWII warbirds and they say power delivery and performance is better than with a single pitch prop. I don't know their reasoning, but that's their experience. Don't know if it would be different with 2- vs 4-stroke gas. Also don't think it is related to difficulties with inverted operation, but wouldn't a velocity stack smooth out engine operation since there's no cowl to block the slipstream?