I just bought a 85 700 off ebay, and the only problem stated is that the tach needle bounces around and reads irregularly. I'm a car mechanic, and I don't know much about bikes (this will be my first). Does the tach get its signal off the ignition coil (-) terminal as a car would, or is it different? I'd assume the tach's probably just shot, but I'm pondering other possibilities also.

I just bought a 85 700 off ebay, and the only problem stated is that the tach needle bounces around and reads irregularly. I'm a car mechanic, and I don't know much about bikes (this will be my first). Does the tach get its signal off the ignition coil (-) terminal as a car would, or is it different? I'd assume the tach's probably just shot, but I'm pondering other possibilities also.
Nitro...welcome!
IIRC, the tach gets it readings from the coils, thru the igniter box which is behind the right side cover, behind the battery.
Are you experiencing any other symptoms?? ...hesitation on the throttle, lack of power in one cyl, or not being able to rev past, say 3000rpms??
Could be the coils or the igniter also....

"I'd assume the tach's probably just shot, but I'm pondering other possibilities also."

You know, strangely enough I have not heard of a Tach being shot.. I've heard of problems in other areas making the tach appear to be wacko though.
One that come to mind is a sticky kickstand switch (Though usually hand-in-hand with this is that bike just stalls when you put it in gear.
I'd check out the coils and see if the problem could be there. Chances are with an 85 you are still looking at the originals..
I'd go with jm1515 on the checking out coils and igniter

So I take it this is like the average Japaneese car ignition system, consisting of a crank/cam sensor, igniter, and coils? Is it a waste-spark system, or are there two seperate coils? As far as other symptoms, I don't have the bike yet, so I dunno. But I'm told it runs just fine, so that's what I'm assuming at the moment.

Two coils, one for front plugs, one for rear plugs

RB

Two coils, one for front plugs, one for rear plugs

RB
Two plugs per cylinder?

Yes.
Everything starts at stator. Two sets of wires come from the stator.
One set goes recitifier/regulator and provides power for bike.
Other set goes to the IC Ignitor which controls the Tach and the spark for the two coils. Each coil sets off two spark plugs. One coil for each cylinder.

So each cylinder has an independent waste spark coil setup... I see, said the blind man... But I guess it's not really a "waste spark" if they both fire the same cylinder at the same time, but same principle.