Almost certainly valve lash and/or burnt valve(s). My friend has one. His was doing the same thing. Adjusted the valves, found the intake valve lash at 0mm (held a bit open). Additionally, rocker not flush to the top of the valve (gap larger on one side than the other). After the valve adjustment it runs a lot better but is still lean (runs better with a bit of choke) and small pops (backfires) when backing off after high rpm. Also took the carb off. All jets clean, carb in good shape, not dirty...no issues there. It now easily runs good enough to ride...but still not quite right. I am 95+pct sure the remaining issue is slightly burnt valve as the valve-to-rocker-not-flush issue, popping, and lean condition are classic indicators of this.
They are notoriously under-powered (heavy to start with). Folks ride them and you get high rpm for extended periods. Over time this results in burnt valves (especially when the high school kids are riding the crap out of it for years).
