Some tach's use what is called an inductive clamp (clips to the plug wire), some hook up to the negative side of the coil. My understanding is that all tach's are not compatible with small engines...for example, my Mac tools digital advance timing light has an inductive clamp, but is only for use on 4, 6, or 8 cylinder engine, on my briggs flattie, it show twice the actual rpm of the engine...i.e. engine speed 1,000 rpm, tach/timing light indicates 2,000rpm. Dig around on ebay, and pick up a used digitron, easy to hook up, plus get cyl. head temp and exhaust temp all in one compact guage!