85-86 had a piston port w/reed assist motor, no exh. power valve, 5spd, shorter front shocks, round rear bearing carrier & 4 bolt rear hubs.
87 was the first year of the full cylinder reed/power valve motor, 6 spd, 87only 8 petal reed, has longer front shocks, 1st year of the p.i.t.a rear chain adjustment, 5stud rear hubs.
88-90 got a different cylinder with a smaller 6 petal reed cage
91-92 have a new rear suspension design.