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MG MGB Technical - Jumpin' Tachometer!

I have a '71 mgb roadster with a Petronix ignition. Ever since I installed it, my tachometer jumps constantly when the car is running. I know I've read something about that, but can anyone refresh my memory on the cause and cure for this? Thanks. Curt
Curt

A 71 would originally have had a current-pulse tach, the pickup of which has a single but complete turn of the ignition feed wire going through it. The 'fix' is said to be to reduce that to half a turn, i.e. the wire passing through the pickup just once and not twice. Unfortunately a 71 probably has the pickup internal to the tach and two bullet connectors on the case (one male one female) which means dismantling as well as removal. Might be easier to get a voltage triggered tach (as used from 73 on), connect the two whites from the harness together (i.e. without going through the tach) and run a new wire from the coil -ve to the single terminal on the new tach.
Paul Hunt 2

This thread was discussed between 02/08/2006 and 03/08/2006

MG MGB Technical index

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