[Steve Carlton]

I'm not much of a technician, but what the hell...

Pulling two plug wires and having no difference implies they're not contributing much. If what you said it true of the rebuild and it used to run good, I get the impression there's maybe stuck rings or maybe stuck valves.

I'd suggest a compression test. If you have a low cylinder(s), put in a little oil into the cylinder and see if it comes up. If it does, maybe mix a little transmission fluid into the oil to unstick the rings?

Otherwise, if you have low compression and oil doesn't help, check the valve adjustment?

I dunno about the tach. My guess is it's unrelated.