The tach on my 914/4 -72 shows 1100rpm when idling at 900rpm (checked with an rpm instrument).
No, it has noting to do with bad points or bad ground etc (i've been there too...), the needle is wery steady and the operation itself seems OK.

Is there a calibration method inside or outside the tach?


/Lars S

These old tachs are not very accurate to begin with. About all that can be done is open them up and use a trim pot in place of a resistor on the input stage to dial them in a little closer. The other thing that might help is to reposition the needle on the shaft a little without affecting the 0 rpm rest point too much. Not sure if there's much you can do to change the waveform off the coil very much, although the tach is sensitive to what that pulse looks like.

Before messing with the tach, make sure your points and condenser are good as electric spikes from the points will fool it.