If the speedometer does not work then make sure that speedometer cable is connected well in its place in the speedometer assembly as well as in the speedometer drive gear. Check if the cable inside is broken or if it has rounded ends in the points of connection to the machine; if so then replace it. Further, it is required to evaluate the state of the speedometer drive gear and change it if required. If the new speedometer does not function properly, use a good speedometer to check proper operation of the assembly; thereafter replace it. If the speedometer is in stable position and you facing the speed display instability at consistent velocities look at the cable way and try to remove the sharp turns and bends as far as possible. In case the sheathing is destroyed, then it is necessary to replace the cable assembly. Check also the speedometer cable by taking out the cable, checking if it contains damages, washing it, greasing it, and placing it again. In case the cable is wrongly positioned, it has to be changed. If the unit is operational only occasionally then it should be ensured that all the cable is connected properly and all fasteners are tightened and the ends of the cable are not worn out or rounded and should be replaced where needed.