[Dave at Pelican Parts]

Actually, one is black with a purple stripe on it. The purple stripe can be tough to see, but it really is there. I usually just think of it as the thinner of the two black wires--but it's a good policy to find that purple stripe just in case.

That wire gets hooked to the same side of the coil (the - connection) as the wire from the points & condensor. Or from the points-replacement gizmo if any.

Also check to make sure the tach itself is plugged in correctly. Check the wiring diagrams available in B&W in Haynes or in color on this very website.

--DD