Thats exactly what mines doing. Sorry about the 2nd gear thing. I noticed too now that the QT shifts when its going. never rode it for real until last night. I played with the carb and it didn't seem to fix much. It was about 9:30 at night when i was running it so i didn't stop much to adjust. Hopefully this weekend I'll take it to a school parking lot and do speed runs and some more slight adjsutments and see if it gets rid of it. To me it sounds like a a or rv limiter.
taking the air filter if it does work doesn't make sense cuz your engine won't last long without it. But how did you cure it? Did you continue to run without the filter or did you do some carb adjustments?