My tach over the last couple of years has been jumping all over the place, getting a lot worse now though. Were about ready to take off on 1200 mile trip next week (with my 85 FLT). Everything has been done to it, however kind of worried about the tach, & (ignition switch's impossibility to get anymore). After doing a search on tach's, I see they pick up off of the coil, my voltage is fine. The tach litterally just jumps all over the place going down the hwy, may read properly for awhile then jump to 4000 for 10 secs or so then jump back down. Sounds like an actual tach issue, no help in the shop manuals, are these hard to find to nowdays?. Appreciate any help, getting harder to maintain these old bikes, I just don't want to buy a new one, & spend that kind of bucks, & have the hassels there having with the new ones!. Love the Evo's!

 

You might just have a loose wire or or bad ground. You might have to pull the tach off & open it up to check inside. Clean up the wires first where they hook to the coil. look for spots where the wires might be wearing off against the frame & grounding out. If that isn't it than your issue is inside the tach shell.
Hatch.

 

Bad ground.
Finding it will be the tricky part. Clean everything everywhere.