Ed,
I wanted to get into this topic in the prop section, but that would not have been very thread specific, would it?
I called Otto the other day and, among other things, asked him if the RCV's 'breathe' any easier than a normal 4-stroke. He said 'yes'. I'm sure you are familiar with the typical valve train of a 4-stroke. The RCV's pull easier and exhaust easier. This is prominent at operating speed. I'm thinking that is why we need to prime the tar out of them. It's not so prominent (the vaccum, pull) at low(hand) speeds as a typical 4- stroke. It needs more priming props turns to get the fuel in there. My theory on the sensitive needle valves is that the motor pulls SO well that it exaggerates any change to the needles. (as compared to typical 4) Any credence here?
Maybe I've been in engine exhaust too long.
Keep us posted on the 'Stang!!
Chip