I had a tachometer that was not working ,then hitting the stop and staying there. Sometimes it would go back to 0 when not running. I pulled it off and tried Marks watchmaker trick with mineral spirits and cotton ball in the sun. It didn't work. I started messing with it while running it with a drill. I found that the threaded part for the cable was a bit loose and when I centered it and held it in place it worked fine. I pulled it apart to see if I could tighten it up.

The drive cable spins the outer cup ( has red dot) which turns the magnetic inner cup that is connected to needle.
The cups were coming in contact with each other.
Looks like the pieces are just pressed together and only a complete disassembly might allow some sort of a fix.
I put some J B weld around the base and its holding it so it works properly.
I had a speedo doing the same thing.
I found a washer that fit just right over the threaded part the comes loose. I used a grinder to make a couple slots for the screws that hold the gauge mechanism to the chrome housing . I used some longer screws to put it back together.

Its working although its not installed on a bike. Its just a spare.

Nice job! Thanks for sharing!

P.S. If I said mineral spirits I misspoke... I'm pretty sure it was kerosene used as the liquid in the old clockmaker's trick I copied and pasted... wouldn't have fixed your mechanical problems, just gently and safely "re-lubes" the gauge.