The good news, is that the engine is running strong (after some dumb wiring mistakes were fixed), and I've run the rear wheel up to 125 or so on the stand (about 9000 RPM / 5th gear, with this gearing). The chain seems to run very smooth and true... in fact, we were all standing around trying to figure out why.
It certainly wasn't what I expected. My best guess is that there is so much chain running straight, that it tends to stay in line because of all the inertia (it looks kinda weird...running like a belt, not a chain). I tried pulling hard rear brake, against 5th gear at almost full throttle...couldn't hold it, but the chain just kept humming along straight, both top and bottom runs?!
Anybody got a better guess?
Baffled, JimL