The speedo on my K100RS worked only intermittently when I bought the bike. No surprise there; they are infamous for giving grief to their owners. In the course of bringing the bike up to what I consider road-worthiness, I opened the instrument, cleaned the connections within with contact cleaner, and put a light smear of silicone grease on them. I also cleaned the sensor in the rear drive and the connector in the wiring about mid-bike. The speedo came back to life. Now, depressingly soon after that, the needle is again falling to zero when the bike is at speed. A loving whack on the left side of the instrument case sometimes wakes the speedo up, sometimes not.
Is this the future of my life with the K-bike? :dunno Does the speedo typically go pear shaped every few months? Jeeze Louise, the Motometer instruments on the Airheads will run for tens of thousands of miles after you "repair" them....