The gear that drives the odometer shaft becomes loose on the shaft and slips. The cure is to slide the gear down the shaft slightly, exposing the contact area. Then you need to "scar" the shaft to provide some grip. Then clean the contact area with a solvent that dissipates (acetone, brakeklean, etc). Then slide the gear back to its place on the shaft and apply one small drop of loctite to the gear so that it flows into the contact area. The hard part is getting to the gear. You need small screwdrivers, steady hands, and good eyesight. I only have 2 of the 3.

You would be better off getting a later model (89 on) cluster so that you can take advantage of the improved lighting design, and maybe a working odometer. They go for $50-$100 on e-bay.