Hi Martin
It sounds very much like you may have a missmatch in you primary gears,MC 28 runs a 24 t crank gear and a 60 t clutch hub gear also a 25 t water pump gear, MC 21 runs 25 t primary,59 t clutch hub gear and a 26 t water pump drive ,
Only the Motor company knows why this was changed ?????,
I wouldnt ride it any more until you check it out,It sounds to me like this could be your problem,
Goodluck and keep us informed what the issue was,
jonmac.
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John