Tach strange and it's not the alt. problem
 

1979 911SC with a tach that seems to have a needle thats seems as though it not dampened or to springy. Hard to describe but when I blip the throttle quickly say to 4000 rpm by ear the needle goes to 6 or 7K on the dial with out the engine actually going to that rpm. Same with the other way when I let off the quick throttle blip the needle bounces down to 0 then settles in to actual idle speed. This symptom is not to be confused with a bad alt issue. I know it's not the alt issue as it will show a steady rpm and will stay there say like when I'm cruising at a set speed. No needle bounce. It only happens when I do a quick throttle blip. The needle just launches and shows a higher than actual rpm. The needle is just springy. Anyone have this issue and have a fix for it?

No ideas? Anyone?

Had something similar and changed spark plug wires. Fixed the bouncing tach. You might do the dark run test: check running engine compartment in total darkness to see if there is arcing around the ignition components. When I did this it looked like Nicola Tesla's lab in Colorado Springs: lightning bolts everywhere. New wires fixed it.

My 78 sc does the same thing. I just thought it was 70's technology.

I installed new wires cap and rotor. I'm wondering if it's something in the tach thats just not quite right.
I wonder if I could borrow the tach from my 87 930? Are they compatible enough to try it with out screwing up either one?

Dies your later tach have the up shift feature?
May not work.

The little anoying light? If thats it then no it dosent have it.

Common problem when they age.

Even listed here on the fifth bullet down

Porsche 911 Service and Restorations

I've got about three tacks that the damping has gone bad, same problem. The cure is as wayner indicates, restoration.

R/Dave

Ok thanks. I was afraid it was in need of repair. Anyone know the cost to fix the tach?

If you have points, check them. My tach was acting funny for about a day, right before the points wore down to a complete close, and the car died on the freeway.
Take you 5 minutes, check the gap. If you have them.