if or tacho is a original one ,this setup that you made dont work very easy.
the original 1200 tachometer is conected in serial whit the power that goes from the ignicion key to the resistor and to the coil.
since you put one relay ,i presumed that you take the original wire from the ignicion key the atack the relay.because of that the tacho measures the relay and not the ignicion coil.
you got to put the tacho in serial whit the relay and the coil to measure the rpm,s
conected the relay to the batery.the output off the relay must pass to the tachometer and then conected to the resistor and to the coil.
this is valid to the original tachometer from 1200 and 120y models.