KZ305 tachometer sticking

Are these units servicable?

Started my bike up for the first time in 30+ years.  Upon startup, noticed that the speedo was stuck at 1-2K RPM and slow to respond.  When you turn the bike off, it takes 10-15 seconds for the RPM to drop from 2K to 0. 

Took it off along with the cable.  Cable is fine and spins free.  Use a drill and you can clearly see the tach struggled to push past 4K RPM and is slow to get there.   Once past 4K, it revs freely based on changing drill RPM.  Stop the drill and the needle falls to 4K and then very slowly sinks to 0.   It's almost like 1-4K RPM the needle is "getting stuck in the mud" and unable to respond quickly. 

Trying to see how this thing opens and it looks like the body is crimped. I don't want to open it up if there's nothing I can do about this.  I'd like to keep the original tach with the bike, but if i need to buy a $40-50 tach off ebay (claimed good/tested) i'll just go that route. 

There are Utube videos on how to open up speedos and techs. The sticky point can probbaly be loosened up. On the other hand, since the bike has been sitting for 30 years, it may just loosen up with use.

Been trying to work it loose by spinning with the drill on a bench, but doesn't seem to make a difference. 

Found a decent video that shows disassemble and the internals of a similar tach. My tach does the same thing but from 4K down and much more sluggish.  At least now i know what to expect.  I'll give it a shot, otherwise just buy a used one on ebay

Has the pinion grease gone bad? Not sure if that would cause the kind of sticking you're experiencing but its all I've got

I've pulled apart a bunch of old speedometer drives and the grease is always petrified and chunky. Pull off the front wheel and clean the drive out. Then pack it with wheel bearing grease and try it again.