[5th_bike]

Wel if it sparks every revolution but only explodes every other revolution, and it counts a spark as one revolution, then it will indicate the right rpm. What I meant to say was "you only HEAR half the rpms".
This tach is constructed for Harleys, and I know that they spark every revolution. I think they actually spark only once every rev for both cylinders, such that the front cylinder gets its spark early (it does most of the work) and the rear cylinder gets its spark late (so it won't run hot).