The tach reads the closing of the points. When the points close, The terminal is grounded, this is what the tack reads. This is also on the primary side of coil. The sparkplugs are fed by the coils secondary side.
Hope the plug wires fix it. If I remember correct, you changed them not that long ago when you were messing with the carb and ignition.
I use standard AC plugs. You really need to change over to a HEI. The engine will run better. Tom
