G'day All,
My 1986 K100RT  (ex police) also known as the Goulburn bike, has developed a problem with the tacho needle fluctuating widely at a constant speed.
So far I have checked that the two long plugs into the cluster are seated properly and that there is continuity in the red/blue wire from the coil to the plug at the cluster.
The other thing may be that the needle is not seated correctly on its pin.
Any thoughts on thiss little issue would very much appreciated.
Cheers
Tom

Pull the sensor out and clean it. It's in the rear drive you need a 4mm Allen key and lever it gently out then check the surface it will probably have some iron residue build up on the face. it comes from normal wear on the gears

Thanks Rick, the speedo needle was also wavering and I have cleaned the sensor which was pretty grotty.
The Tacho needle is the problem. There is continuity for the red/blue wire from coil to plug and continuity for the earth wires from plug to frame.
Before I delve any further I'll for any other replies.
Cheers
Tom

The tacho picks it's signal from one of the coils. It could be a faulty/dirty connection.
Regards Martin.

Thanks Martin,
The coils are new and the red/blue wire ( the signal sender from the coil) connection is tight and clean.
Cheers
Tom

If the needle gets loose it will fall off completely so that is not your issue.

Connection where the connector on the back of the cluster connects to the pin?  Make sure that the female terminal in the connector has not been pushed in.

Also try to trace the tach wire inside the cluster to see if you can find a flaky connection there.

Thanks Duck,
Opening the cluster is my next step. I'll keep you posted. 
Thank you all for your input.
Cheers
Tom

The other thing that I have seen is the wee coil spring wrapped all out of shape. A gentle dig with a jeweller's screwdriver sorted that and stopped the needle bounce (speedo, as it happens).

Is there any misfiring?  A faulty Hall Sensor signal could be causing the problem.

Thanks Dai and Point-Seven-Five,,
There is no misfiring. The engine is running as sweet as a nut, just ask Waz.
I'll be delving into the cluster today and check out Dai's wee coil spring. I'll keep you posted.
Cheers
Tom

Does the fluctuating ever settle down or is it constant.
Regards Martin.

Hi Martin, the fluctuation is pretty constant. Even at idle the needle moves around a lot more that normal.
Cheers
Tom