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I have a ‘79 kz750, the speedometer cable was missing when I bought it. I bought a replacement, greases it up and installed it but now the speedometer doesn’t really get above 20. Is the cable installed wrong or is the gauge itself broken? 

Sounds like a speedometer problem - have you tried tapping it while you're going faster than 20? Mine sticks sometimes and tapping will temporarily remedy it. 

I’ll order a new one and we’ll see if that does it