Those Aqua Meters are a sweet tach. Old, but they have a jeweled movement inside, I believe. Yours is the later model with the switch on the back. It should work fine in your application. Possible your rectifier is shot and not sending the correct signal to the tach. You could try another tach as a test. Do you know if the engine charges the battery correctly when running? Do you have a voltmeter on the dash? If not, check the voltage across the battery when running with a normal voltmeter.