My DS7 had a tach that worked great but had rust on dial. So I bought a nicer one as-is  and needle shoots all over the place and always in red.  Can I squirt something into this and avoid buying another?

I doubt its that simple but be good if it is. Think its all done by magic magnetism. Might've broken the tension spring that holds agains the magnetism or have a bent shaft or bent column so coming at the internals at an angle. I just made all that up but did look inside 50 yeaers ago.
Think DEET is the man, or if not someone will say soon. Can you return it or not worth the hastle.

thanks RT
I bought "as-is" to get a cheap price, my thought being that seller has no clue whether tach works or not. I took one apart once to fix glass and it was a nightmare. My thought is that specialized can opener is needed to do this properly. Think I will try something radical, like submerging in alcohol and if this doesnt help put good one with bit of rust on dial back on and just chuck 'nice one" in trash or sell it on as needs repair for a few bucks

Kerosene on a cotton ball stuck into light bulb opening(s) with tach face always up (if kerosene gets inside it will stain the face).  Let it sit out in sun face up for at least a day. 

This is an old clockmakers trick shared on an old 2 stroke forum.  the kerosene will evaporate up into gauge and soften/"reconstitute" the old grease. 

Filling will alcohol with probably ruin it. 

thanks Mark, I am going to give that a shot!

is BBQ lighter fluid close enough to kerosine? or should I go buy some?

Kerosene is an oily fluid. I've only used the real stuff. I have an old kerosene lamp and a kerosene heater though. They used to sell it at Home Depot by the quart, gallon, and five gallon. Walmart and other stores have it sometimes too. Make sure you get real kerosene, not the substitute stuff they sell.

If I couldn't find kerosene (or it was too expensive) I'd try diesel fuel or Jet A.  You could probably get a few drops out of hose at station...  enough to soak cotton ball. 

thanks again, will do