New vintage airguide to '61 75 johnny....green & black wires. When wired to leads at dash it maxes out immediatly. I had the same result when I connected a short lead to the coil where the wire goes up to the points, the other I grounded to the block.<br />The diag I have shows it getting its ground from the ignition light and the other making its way thru the box to the same side of the coil. <br />Now here's where the curve ball is thown. The tach came in the original box matched by model number. The box has a label stating an application chart for this model, below is has 2 1971 mercury models and several more back to 1968. Could this be designed to work for those motors and not a 60's johnson V4.
<br /><br />All I can tell you is what the instructions from a 40 year old manual said, and that referred to a 40hp motor which should be somewhat similar to yours. I beleive the "M" terminals on the ign. switch actually connect to the breaker points under the flywheel if that's what you are asking.<br /><br />There should be 2 terminals on the back of the tach (not inside), one will be marked with -, the other a +, also keep in mind you have a tach with inst. for a Merc. The inst. I gave are for a 40hp Johnson, they may work then again they may not.<br /><br />Good Luck