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1980 CB750 Speedo Popping

Intense951

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I’ve broken my second speedometer on 1980 CB750c. It’s an aftermarket LED style one from Vintagecb750. I bought the bike with it already installed and when I popped the first one I got a replacement and that one popped after about 100 miles. Basically what is happening is that the speedo still counts miles but the needle doesn’t work. When I back the bike up, I can get the needle to back itself off to zero until I accelerate and then it goes way above 140 (quickly) and doesn’t read speed. But the miles still count. When the first speedo went I thought I heard a spring snap but I was riding at highway speeds with earplugs in so not really sure.

Vintage says that they have an extremely low failure rate of the speedos and thinks that there’s an under laying issue. The whole system seems pretty simple; gear at wheel, cable and then speedo. The speedo is still counting miles so it can’t be the gear (or is it?) and I already replaced the cable.

Thoughts on what can be happening here?
 
A cheap crap flyball governor used there, it is separate from the mileage gearing. A flywheel with weak magnet spins under a needle mounted on a super weak spring to unwind when the needle gets pulled by the spinning magnet on drum inside it. Faster the magnet spins the more the needle pulls over to move speedo rate. Easily broken, it's a wonder they work so long to begin with, simply messing with the spring with a finger can screw one up as can dropping one only a few inches.

If any vibration absorption has been altered there, that can mess them up.
 
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