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MG MGB Technical - Speedo/Tach Help

After a three year journey restoring and converting my 1980 B to a V8, the last two piece have stymied me and my mechanic, the speedometer and tachometer.

I have a 3.5 litre Buick V8, with a Borg Warner T-5 transmission.

My mechanic seems to have tried everything, but the speedo is still off by 15% - 20% and now the tach which use to be ok in now off by 10,000 rpms.

I'm looking for a shop in the Northeast, preferable the New York Metro area that can do this work and get it right.

Any suggestions would be appreciated.

Thanks

Richard Cohen

ricmgb

Hi Richard,

I can't help your with a shop in NY, but I can give you hints about the problem.

The tach needs to be re-calibrated, as the V8 generates twice the pulses of a $ cylinder, therefore would read twice the actual revs.

The speedo is wrong because the speedo drive in the transmission is a different ratio to the original transmission. What will need to be done is either rebuild the speedo, with different gears internally, or to fit a ratio changer, which is a small gearbox that is connected to the speedo cable, between the speedo and the transmission.

Herb
Herb Adler

Richard. Do you mean the tacho is off by 1000 RPM, they don't go to 10,000 do they?

If all else is equal you should be reading twice the engine revs on the tacho as noted by Herb. If you have measured engine revs with a hand held tacho and the car tacho is 1000 revs out, and it worked fine before, it would seem the ignition circuit on the V8 is not suited to the tacho.

Tony
Tony Oliver

There should be a variable resistor for calibration, or at least there is in the earlier RVI. A change in value of the appropriate timing resistor will also do the job, although I don't have details for the RVC (Herb recently posted it for the RVI).
PaulH Solihull

Nissongers. RAY
rjm RAY

This thread was discussed between 06/07/2011 and 07/07/2011

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