Autogauge Tach

A tach does take some energy from ignition circuit. An estimate of that, could be measured with an Ohm meter, by measuring the disconnected tach lead to ground ignition off. I would expect about 100k Ohms would be good.

It is also possible that there is a voltage drop in the ignition circuit, making it weak. Adding the tach, lowers the HV ignition enough to make it no start. So measure the voltage at the coil + terminal to ground, engine running. If the circuit, has a ballast resistor, it should measure about 7V.