The tach consistently stays at zero in cold weather, sometimes even after just sitting in the shade for a while. Will jump up and start working once warmed up. I doubt it's the coils as it works fine when it's warm but I do have an extra set I can test if needed.

 

mine does this too sometimes, and i feel like its probably a bad capacitor or something like that.

not sure what the easiest way to figure this out, there is a test plug you could plug a tach into, and see if its the tach or the ignition. or you just try another tach