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MG MGB Technical - Tacho reading low?

Just converted my 66B to negative earth after holding out for many years. Changed the tacho to a later voltage type, not been able to check accurately but it seems to read at about half speed. Have I done something silly? Car is fairly standard otherwise, no eletronic ignition etc.

John
John U

Hi.

Is it possible that you have a tacho for a 6 or 8 cylinder car ?... this would cause a low reading.

You might like to take a look here :

http://www.mgcars.org.uk/electrical/rvc.html

Don
Don


Thanks Don, a useful site. The tach is marked 4 cyl with RVC2415 code. Believe from a late model B.

I've wired as per Paul Hunt's info but wondered if I had missed something obvious. It may just be that the tach is out of calibration. Have to give some thought on how to check it.

John
John U

John - If you can get hold of a good diagnostic tach, such as the one that is sold under the gunson brand name, hook it up and lay it on the windscreen and compare the readings to the installed tach. Note that the installed tachs are not real linear in their readings, so I always set mine at 3000rpm. There is an adjustment pot inside the tach that can be adjusted to bring the readings into where they should be. Good luck - Dave
David DuBois

John, you might want to check the clearance on your ignition points. IIRC the type tach you are using relies on the charging of a capacitor and the dwell of the ignition can affect that charging rate. Shouldn't be as much difference as you are seeing, but something to check on that might help.
Bill Young


David, Bill,

Will think about the Gunson tach, must be useful when tuning.

Seem to remember something about points in the info I looked up. Should check that first. The car needs a good service anyway, just did the Corsica trip with the MGOC! Some heavy mileage.

Regards,

John
John U

John, just use the original tach and change the wiring per Moss Motors suggestions. Reasonabley straight forward and works well.

Andy
Andy Preston

Andy,

Would have used the original but it was sticking at higher revs. Neighbour was a watchmaker and tried a few things but none worked. Guess it is just too old! Seemed a good time to convert.

Been sidetracked again, rear brake cylinder leaking. Just removed it tonight. Good excuse for a clean up round the back axle.

John
John U

This thread was discussed between 10/07/2005 and 12/07/2005

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