So my Tacho is running wild !.... needle moves left to right and right to left, like a fan as soon as I open the throttle.
 (At idle speed its stable and show the average 2000 rpm)
I connected a drill to it and that seems to work normal at different speed without any fluctuation.
Although sometime when engine stop the needle seems to stay blocked on the 2000 rpm mark... a little tap on the side of tacho and needle drop to 0.

So I assume problem is likely to be the gear ?  ( Drive or Driven ?). 
Any suggestions before I take it apart ?
Thanks
Bernard.

 

Nope. More than likely  a broken tach cable,  if not,  the problem is in the tacho head.  Probably somewhere in the needle damping system.  Bit of dirt or rust in the damping grease or suchlike.

Edit. Would have said pretty deffo, the cable, apart from your comment about the needle sticking with engine stopped.

Debris stuck between cup and magnet yoke, in the head.

cliff

TinKicker.... Thanks for the answer. Cable is not broken and square both ends.
Cliff..... Yes debris inside is possibly one problem, but why would the needle be going stable from 2000 to 10000 when attached to electric drill ?... The needle goes flying all over left to right./ right to left very rapidly when connected to gear drive.
I will open the tacho to clean inside it as you suggested anyway..

If sounds like this is a recent occurrence? Did anything happen that could have affected the operation?

I'd try turning the handlebars from one side to the other and see if it affects the operation.

Any chance the cable housing got snagged and stretched due to a tree branch or fall?

My thought is that the inner cable is "skipping" in the drive due to poor engagement... as in inner cable might be just slightly too short?

Also, was the inner cable inserted into the outer housing from the bottom to the top?

Spinning mag yoke and alum cup has interference which makes the yoke stick to the cup and sends needle flying with the cable.
Then not, swinging opposite due to return spring.

Cable is attached to yoke(spinning magnet) Aluminum cup is attached to needle.

Same principle as a balance damper or magnet dropped thru a copper tube.

Ask eddy current. Lorentz knows him well.

cliff

Is the drive part the cable screws to slightly loose in the tacho body from damage or had a bump, then when cable is screwed on the pulling of the cable upsets the alignment with the magnet thingy.