Plugging a tacho on the two-strokes was not an issue a few years ago. On the other hand modern four-strokes have the coils built in in spark plugs. Therefore, the strong signal is right there. According to Chapin the tacho wire has to be put down there. You remove one of the plugs and by using adhesive tape for electrical applications you attach the wire on the plug/coil as it shown on picture. By using a small cutter carefully shape the elastic seal on top in order to let the wire pass through it without leaving too much of a gap. In other words you make an inductive coil therefore, you get a better reading.
