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MG MGB Technical - Speedo & tacho fixed

My '67 Roadster's speedo and rev counter have been providers of misinformation and propaganda for years. Not being sure this is a valid defence against speeding tickets, I finally got round to fixing them.

Tacho: Increasing/variable reading solved by replacing the 0.22uF capacitor, following the instructions at www.classictiger.com/techtips/motach.html and http://www.mgb-stuff.org.uk/ . Doddle to do, but confused myself by first trying the fix with a later tach which has a different circuit. New cap left the reading consistent but low. Not surprising as it replaces an obsolete cap of different value. Calibration was also easy, done by twiddling the knob at the back through a hole drilled in the case when it was in bits. Having no tacho on my multimeter I set it by ear then read the archives and found a clever method using a battery charger. http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=97&subjectar=97&thread=2000011502071116561
This should send a perfect 3000rpm signal by putting a headlamp and the tach in series with the charger. I connected it all and the tach came to life with a reading of about 2400rpm so I turned the calibration up some more. Took it out on the road and it sounded like it was reading high. Without another reference I decided the only way to check it was to match revs against road speed. Time to fix the speedo.

Speedo: Came out of the dash OK but I couldn't fathom how to liberate the internals from the case. The rod for the trip meter was too long to allow everything to push forward. It must have gone in originally, but being in a hurry I filed the hole the rod goes through until it would clear the case. This is a BAD thing to do as there are magnets inside that now will undoubtedly be fouled with filings. Oh well. I knew from timing my speed between yellow phones on the motorway, and checking against a modern car that the speedo was 10mph slow at 70mph. I followed instructions found in the archives to hold the needle away from its stop and turn the large metal balance ring at the back of the mechanism. I found that about 1mm turn of the ring (clockwise as you look at it from the back of the speedo) upped the reading by 1mph at 60mph. Confirmed the accuracy with another timed run. All well.

Back to the tach: Fixed speedo showed the tacho to be reading high. Gave in and bought another multimeter. This confirmed that the perfect 3000rpm setting from the battery charger was actually nearer 2350rpm. This bugs me because I don't understand why the charger method almost works but doesn't but I'm pleased both instruments are sorted and will leave it at that.

Thanks for all the help in the archives. This site is an incredible resource.
Steve Postins

Steve, there's no doubt that dozens, possibly hundreds of us will look back through the archives in future years to rely on this post.Thank you!
Jeff Schlemmer

This thread was discussed on 29/03/2004

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.