86 had different linkage and longer shock, different cylinder(either the rod or stroke was diff. dont remember which) different front motor mounts, 86 and 87 had the same rear motormounts.
87 had new linkage ratio and untill mid year used the 86 shock with an internal spacer, mid year it got a different shock. cylinder and (stroke or rod?) changed. clutch update on later 87's I believe (thrust bearing setup on pressure plate) front motor mounts changed, airboot resonator....
88 got a 1" shorter aluminum swingarm requiring the new rear motormount. cylinder porting very minorly changed a lot of people swear the 88 motor was the best. headlight integrated into nose plastic and footpegs moved
89 got different colored plastic
Theres probably a few other differences and I may be wrong on a thing or two, its been a long time since I thought about it. any year 250r is a great bike, the 88 was the nicest in my opinion, but it had an alum swingarm, that alum swinger is now 20 years old, so it should be looked at cloesly.