kz750N tach issues

I have an 82 750 Spectre,(N) is the Vin model designation. I replaced the coils and wires last year with a set of used Green Dyna coils, and new dyna 7mm wires. The bike continues to run good, but the tach does not indicate proper RPM. It is a electronic signal tach, not a mechanical cable type. Any Ideas what would be causing this? Are the green coils not the correct OHM's? thanks guys.

I'd check the wiring from the BLACK wire going to the ignition coil(on my 82 GPz1100 B2 it's the left coil)from the tach. If your bike is like mine the tach serves as a volt meter when a button is pushed.