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MG MGA - Tach question - Electric conversion

Maybe a simple question.

I just bought an 'A that has a 'B engine installed. There's also a Smiths electric tach installed. When I start the car, it settles down to a nice smooth idle. If I had my eyes closed, I'd guess 800-900 RPM. But the tach reads around 2000 RPM! And when I blip the throttle, the tach needle swings rapidly all the way upto the stop at 7000 RPM. The gauge is marked 4 cylinder, Positive earth. Don't have the car here, or I'd give the #s. Now I must admit, I haven't had the time to check to see if the car has been converted to -VE earth. What symptoms would I see if the car was -VE with a +VE tach? If that's not it, any other ideas. I haven't had time to try another tach, either.

TTFN
Derek Nicholson

I used a Jaguar tack face mounted unto a Elec tack. The needle had to be shaved where it mounts as it was too heavy. When you look at the tack, it looks original with original needle. I sent it to Mo-Ma and they put in a new tack board and calibrated it. It looks original and works great with my 18v engine.
JEFF BECKER

Derek,
There's a trim pot inside the tach for calibration. I'll try to locate the web site and post back.
D Sjostrom

Here's the web site that goes into repair & calibration.

http://www.classictiger.com/techtips/motach.html
D Sjostrom

Some electronic ignition conversions are not compatable with some electronic tachometers. If you have an electronic ignition installed on your engine this could be the problem.
Ed Bell

I haven't taken the distributor cap off to see if there's a Pertonix installed, though I can confirm that there's not a V* installed and responsible for the tach reading around double what it should.
Derek Nicholson

This thread was discussed on 08/07/2006

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