Jumpy tach
 

1966, 912, tach starts jumping around at 1500+ rpm and will whip between 1000 and 7000 while driving at slow speeds but is more consistent to normal at higher speeds. Is this the tach, the cable, or the ?.

Roger

The tach is electric. You might try cleaning all the contacts/connectors at the instrument, coil, and distributor. If this doesn't work. Then the tach will need to be rebuilt. Of course the points and condenser are in good shape.

My jumping tach was resolved when I replaced the voltage regulator.