Another possibility is the drive at the wheel. I had that go bad and made the needle jump, it would lose drive then catch and lose it again. The "ears" that engage with the hub wore out. I did eventually lost drive all together. I had the same thing happen. It was a easy fix, nothing wrong with the speedometer itself, but the bad drive made it go crazy.
05 speedmaster - 1100cc, 11:1 racing pistons, Carillo rods, thunderbike cams, ported and polished head, 2mm over intake and exhaust valves, Barnett kevlar clutch, scepter pipes, oversize manifolds, 45mm HSR's, TTP stage 4 firestarter
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