Had same problem about 500 miles into my bike. Before the first tank of gas I had a magneto fail, and after the bike died on me again suddenly, I blamed the mag. Got spark on new mag, but as hard as I peddled, bike would barely run.
Went to Motorcycle Mechanics Institute WAY back in "91, so here is the list of what I checked
1) with mag cover off, turn motor over with a drill and a 14mm socket and check spark
2) turn off fuel at petcock, drain float bowl and spray carb cleaner with throttle open and drill start motor to check a fuel issue.
3) check compression- aha! compression was up and down intermittent and I was really scratching my head, easy enough to pull head and I saw some blow
by evidence around the gasket and seating area. So, I re surfaced the head with some 400 grit sandpaper on a glass table for a true flat surface and set head back. Got bike to start a little better but would still not hold throttle without peddle power.
4) took head back off and on close inspection saw a defect on top edge on piston - again AHA!!- time to remove cylinder, very easy to do and then I found the culprit. bottom compression ring had broken off and chiped the piston, also chipping the cylinder and scoring it. the cerclip holding the wrist pin also broke a piece off and gouged down the side of the piston- just enough to still run, but with enough blow by not to make a complete burn.
Cause.. I believe vibration at high rpm especially with throttle open on down hill- just to much for the iron ring and clip and something had to give.
Moral is DON'T EVER GIVE UP! keep digging there will be an answer
RIDE ON !!
