MG Car Repair: 1968 mgb tach, thumb screws, tiny variations

Question
My 1968 mgb tach seems to work just fine except when the engine is reved up slightly the tach reads a lot higher than the engine is actually turning. At 50 mph the tach reads 4500 to 5000 rpm. Speedo seems to be OK. The engine is a 1972 mgb. Any ideas greatly appreciated. Thanks,Nate

Answer
Hi Nate.  The 68 MGB had a strange electronic tacho with a loop of wire going through a horseshoe on its way to the power side of the coil.  As the ignition fires, the tacho measures tiny variations in the supply to the coil.  Either you are getting multiple sparks, or the tacho is faulty.  The speedo is a mechanical unit, so it would not be affected.  

Does it read under 1,000 rpm at tickover?  If it reads more, then the needle could have slipped round on its spindle.  

Has it always been like this, or has the fault developed recently?  If it is a recent problem, what else have you done to it recently?

Is the tacho properly earthed?  There should be a thin black wire attached to one of the holding thumb-screws at the rear.

Please let me know what it was when you find it.