I am going through the same things. Just put the new ported motor together with PWK 33 s and have the same problem. I will try the change of octane. I have jetted and jetted and still have it. I have changed the stator,reed cages , reeds, intake, coil , and cdi with no fix. i have put different carbs on that are jetted for the same motor and no fix. Fuel octane could make sense.
let me know what you find out. I will do the same