Wow ! thats bad!
You ask if it is Worth restoring? I must sincerely answer your question with questions.
How much trouble are willing to go through to try?
How much are you willing to spend to try?
How much will you spend before you kick yourself in the A*s and toss it in a junkpile?
None them questions are intended to receive an actual answer, but rather meant to provoke your own thoughts.
The last busted Honda tach and speedo set I had open (out of curiosity with no intentions to try,to save them) were pretty skunked from rust and busted lenses, faded/bent faces missing needle and rust through broken spring on the speedo ...ect.
What I can tell you is that don't really act like the resisting spring is dead because how quickly it drops back down.
I think You probably have chunks of rusty crud in between the bell and magnet. HERE is a link to something informational about tearing them down and POSSIBLY rebuilding the guts for both a Speedo & a Tach.
Granted they are for a Yamaha, but all old cable drives are built in the same exact method, just different math, sizes, and angles from one to the next.
There is also small chance (if you haven't checked yet) that the core of your tach cable itself is not turning well, maybe dry rusty or even a greasy mix of both....if it hitches then snaps free repeatedly, it could cause the erratic needle behavior. I say check your tach drive cable before, You considering ripping into the tach housing. A Tach cable is easier to get at, and a bit easier to replace So if the tach isn't broke yet why break it?
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