ballast resistor or not, shouldnt affect the tacho. I had the same tacho on a hilux 22r motor (electronic ignition, no ballast) KE25 (points, ballast resistor) my WB One Tonner (HEI, no ballast resistor) and for a short time my ke30 (points, ballast resistor). I have a feeling theres something not right with the signal its getting.