take the tach off the mount and shake it gently. If you hear the weight bouncing around you know it needs to be glued back to the round bracket. Since the tach needle is stuck high on the scale, I would give a good bet it's the simple counterweight. Easy fix, but not easy to take apart & reassemble.
If it doesn't rattle, then it might be the windings.
I was lucky enough to fix mine. I took a red magic marker to the needle.