MG Car Repair: MG Midget Tacho calibration, double pulse, alternator belt

Question
The tachometer in my 69 midget is reading much higher than it should be. It seems to be about double that of the actual engine speed. The wiring is correct, the white wire in on the +ve side of the coil and the rest seems to be in order. Is there any way I re calibrate it, or should I look for another electrical problem.

Thanks

John

Answer
Hi John,
Unless you have a double ignition pulse the problem has to be in the tach itself. Find a shop with a scope to test the ignition primary circuit for a double pulse and if that is ok all I know to do is to get another tach.

I have reversed polarity in the earlier tachs but never tried to calibrate one. One other thing you might check is charge rate with a voltmeter across the battery. A high charge rate might make a tach read wrong (should be 13.8 to 14.5v at a fast idle)Also, as a test remove the alternator belt and start the engine to see if a leaking diode in the alternator could be a cause. I did run into a bad diode in an alternator that would drive a tach crazy but it didn't just make it read wrong.
Howard