weird ignition issue after installing tach

You need a multi-meter to figure this out ($3 at Harbour Freight, sometimes free). Measure resistance from the tach case to its gnd wire. If <5 ohm, they are connected. If the case is isolated, then something is bad in the tach. Measure resistance from tach in to gnd. It should be >1000 ohm. My guess is it is much less and is shorting coil- by hooking it up. Not sure how it can work with 1 wire connected, but it is an AC signal so can jump a gap (capacitive coupling).