1970T Tach issues

[ARCSinAK]

Any thought on possible causes of jumping needle on my tach? It seems to work on and off, but often will jump a couple of grand randomly while at various speeds. I have recently rebuilt with alternator but it functions normally for months after the Alt rebuild.
I thinking maybe a voltage issue but I do not have any other electrical issues. Maybe a CDI problem. Your thoughts are welcomed.

Regards-

JS

[sepia2.4]

Check your distributor points. I've had that cause tach issues on my 72.

[gamin]

Points are the usual culprit with those symptoms.

[ARCSinAK]

Thanks for that advice, I put a new set in maybe they shifted position. Would that symptom be caused by worn out points or in correct points gap?

[Porboynz]

It can also be the tach itself, the capacitors and other components in it go out of spec. Mine drove me crazy doing the same thing so in the end I had an instrument repair shop fit the mechanism from another 6 cylinder car, been smooth as ever since. Looks the same, retained the stock needle etc, probably a bit more damped that the original which was very lively, but better than on that jumps all over the dial and catches your eye. Its an easy swap if you have a friend you can exchange gauges with for a weekend to see if the fault moves between cars.