Hi

I have an NC21 and the tacho seems to get rather erratic after 10,000rpm to the point I'm not actually sure what the true revs are...
It seems to do it in all the gears and it seems fine below 10k.

I take it the signal is picked up from the coils so my initial thought is that perhaps I should perhaps start there or am I barking up the wrong tree and should maybe be looking at the clocks...?

Any ideas fellas?

I have the same issue on my nc23, in fact at times it bounces all over the place so i dont know what revs at all i am doing.
I asked and was told it was prob an earth. havent sorted it though.

Yup, most likely a bad earth. Higher RPM the more the bike will vibrate and shake the earth contact about a bit. Check that first and see how you get on.