If the tach works with a higher 'spark' value ignition box, your R1 resistor (first off input) may have gone south, giving it a higher resistance than designed.
I've had the tach apart to resolder everything I could get to in an attempt to 'fix' it. No reason I'd recall a resistor on the signal input line...wasn't looking for such a thing.
What I do recall being inside the tach: A LOT of flatpack ICs! I could not have guessed at such a complicated setup for a tach. It is a shift indicator tach with an electronic set-point. That adds some circuitry. Still, the thing looks more like a computer processor than a tachometer.
I'll make a note of your resistor input notes on my tach paperwork. Next time I have it apart, I'll check on it.
