The tachometer needle on my NC21 is moving around erratically. It used to work fine, but recently changed. The engine is running fine. Any suggestions for troubleshooting? We did check the cable and everything seems tight.

Tacho faulty, test with known functional unit, ignition unit faulty, test with known functional unit or loom/connector fault, inspect connectors and check continuity of loom. Any other approach requires testing requiring specialized equipment.

My tacho played up, reading two or three times actual engine speed, then stopped working. I traced the fault to a bad earth connection on the back of the instrument. That was for an NC30, no idea if a 21 is the same, but thought it worth mentioning.
John

We finally got around to working on this. We took the front fairing off and disconnected all the related wiring connections. We used a bit of sandpaper where it fit, some electrical cleaner and grease, then reassembled and the problem seems to be gone!