Original tachometer troubles

Dunno man, just bumpin' you back up the list.

Not sure how the stock style tach is wired, but say a simple store bought tach would trigger off the coil. If the car dies when you attach the ground wire for the tach, then the tach must be shorted internally? It sounds to me like you are grounding the ignition and the tach is the only thing between. Assuming it's wired corrrectly of course. Do this if you can. Borrow or buy a cheap tach and try it out using the same wiring. Just extend the existing leads temporarily to see if a different tach works ok. If it does, then I would guess the new tach is defective or failed in some way. If it still dies, then that would be a good problem. I would have to look over the hook up again.