Well cross your fingers and hope thats all it possibly is, that you have too much oil in it...and NO, the older more is better theory does not apply to overfilling the crankcase on a bike!!!! Look in an owners manual or shop manual or call your local bike shop and find out exactly how much oil it should take, and only put that much in it. I don't remember if those older XR's had the oil sight glass in the side of the crankcase or not. but if it does and you can't see the top of the oil mark in the glass, all sorts of goofy things are going to possibly happen (as you might have already proven )

The chain rubbing the shifter still has me somewhat baffled though. It will be interesting to see what size sprockets are on the bike....it could also be other possibilities but will wait to hear back on teeth count.

good luck!!!