Well, a basic understanding of what may be happening might help.
The Tach reads or accepts the pulse from the distributor for each negative portion of the coil discharge. Or, in other words, when the distributor sends the grounding signal to the coil, the tach counts a pulse.
If you have a CD system with multi-strike ( more than one discharge ) firing, you can get more than one pulse per cylinder making the tach read higher than it should. CD system have an "ON" cycle signal designated for the Tach. You will need to use it.
If you have a bundle harness ( wiring that is all in a bundle and then fed to the engine ), you may be getting noise to the point that the noise itself is adding pulses to the tach reading. A quick fix for this is to run a shielded line between the tach and the tach signal. Ground the shield to the block/chassis.
If you have a double strike firing system ( two strikes per cylinder ), you will need to find out how to provide a single pulse for each. I used a conditional Flip Flop once that worked fine. I think it all depends on your system. The distributor supplier should have a fix.
Well, I hope this helps.
