Re: speedifour rpm gauge.
There are two black wires connected to the "M" terminals on the key switch. One goes to the post on the side of the magneto, and the other one is grounded at the powerhead. The tachometer connects to the one that goes to the magneto. Simple way to tell which one that is....try connecting to one, and if it doesn't work, must be the other one. 50% odds ain't too bad.
Having said all that, the motor requires a tachometer for a magneto motor. Nobody makes them any more. Modern tachs won't work on it. One exception is the Tiny Tach digital tach that gets its signal from a spark plug wire.
