The tachometer has a sending unit that should be located over the flywheel. The unit will have wires that plug into it. Carefully remove the wires one at a time. Note or mark wire locations. Carefully clean the connections with a small piece of scotchbrite pad or emery cloth. Spray the connections on the unit and where the wires plug in with WD40. Reconnect and see if this corrects the problem. If this does not help then you will need to test the sending unit. Test as follows. Remove and label the wire connections, then using a multimeter ,measure the resistance of the sender. The resistance should be 1.6 kilohms. If the resistance is incorrect, replace the sending unit. If the resistance is correct then check the wires leading from the sending unit to the tachometer. If everything checks out, then the gauge is probably faulty.
