check to see if ur sprocket is messed up, if not the sprocket it could be an internal gear
NEVER hold the rear break with ur thumb still on the throttle, cuz the engine is trying to pump and turn the tires but if the breaks lock up the rear end the front sprocket and gears cant pull the chain and thats f*cking up the inside of the engine. if u want to use the break like ur going into a turn always pull the clutch before you use the break that way you maitain high rmp but you dont f*ck the engine up.
if thats not what your talking about maybe this will help
your clutch is slipping and needs to be replaces (50$)
your clutch cable has too much play and needs to be lubed and adjusted
get a new clutch cable if ur still having problems after you fully adjust it.