Good evening all.
I have a 1988 K100RS 8 Valve with 70K on the clock. It has served me well and I have done 30k on it since 2011. It is my first K and lives outside under a cover.
Recently I have noticed that when riding it under 3000 rpm in the fuel gauge and temp gauge react normally. However over 3500 rpm in the fuel gauge reads less fuel and the temp gauge starts swinging left to right quickly then moves over to the left and stays there. However if I throttle back to under 3000 they both start reacting normally again. This means you have to slow down to 55mph to check the fuel and temp of the bike?
I did find the positive post on the battery slightly loose. The alternator is charging at 13.8 volts and the battery reads 12.6 volts with everything switched off.
I have looked around for loose or corroded connections but all is clean and tight.
Has anyone had anything like this in the past.

Regards

Blacknails

Both accessory instruments share the power (Green/black), ground (Brown) and instrument lights (Grey/blue) wires coming off the additional instruments connector on the harness.  This connector is located under the tank, on the right side towards the middle of the bike and looks like an old fashion PC hard disk drive connector.  This type of connector is notoriously unreliable because the circular pins loose their springiness over time.  I'd imagine that this will be the source of the problem - poor connections between socket and plug at this connection point.  The chances that both instruments have decided to become intermittent simultaneously is unlikely.  

Check the cleanliness, and connection reliability at this point in the harness.  If you measure the voltage between Green/black and brown at the connector itself while wiggling it, a bad connection might show up on your meter.

http://technik.flyingbrick.de/images/6/6b/61_Instrumententraeger.pdf