A jumpy tach usually means problems in the ignition LT circuit. 73 MGBs
have a voltage pulse operated tach, signified by 'RVC' on the face plate
(earlier current pulse types have 'RVI') and with this type problems with
any type of ignition, including points, factory electronic ignition, and
aftermarket electronic ignition systems of any type, can cause misfiring and
jumping tachs. Check the 12v supply at the coil and electronic ignition and
make sure it is steady when the tach jumping and misfiring occurs, if not
it is probably bad connections in this 12v supply that is causing the
problem. Another common cause is a broken or frayed ground wire in the
distributor, and this affects points ignition and some aftermarket systems
like Pertronix, but not usually those systems with just an optical or
magnetic pickup in the distributor and the rest of the electronics outside.
Other than that any loose connections in the circuit will cause problems,
and of course problems within the electronic modules. For this last
substitution is the only diagnosis, and substitution with points and
condenser is best as these can be diagnosed simply with a voltmeter.
----- Original Message -----
> I have a 73 Midget, no pollution stuff, a very mild cam , electronic
> ignition, and a weber carb. Ever since moving to CO from MN I cannot
make this car
> run. I have tried almost everything. It idles smooth at 1000 rpm but as
> as I put a load on it over 1500 RPM the tach starts jumping all over and
> engine misses and struggles to gain power. Any ideas?