I just got this bike. And I took it out for a ride and when I get into 4th gear and accelerant a lot it won't stay in 4th it revs up like the clutch is being pulled in and the throttle stays where it is reved and it slips down to 3rd. Won't stay in 4th. That's the only hear it does it in. Anyone know what it could be? Not sure if previous owner did the oil right. Is it a major expensive fix? Thanks!