The square end of of the cable should go into a square hole in the speedo drive. If you undo the cable from the speedo end, push the cable down into the drive and then try spinning the rear wheel. If the cable rotates then its just a matter of the cable being too short. If it doesnt rotate then take the drive off ( the 2 allen bolts) and make sure the plastic socket is still there (its white, well it was when it was new) that is driven by the gear box sprocket bolt. If it is, turn it by hand. If the cable still doesnt rotate then its the speedo drive. They are pretty common so you shouldnt have to much trouble getting one if its ducked.
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If I have to take the carbs off once more...