[THEFERMANATOR]

The tach normally does not run off the ignition on that engine, it runs off the stators pulse for the charging side. Sounds to me like a magnet issue or stator as that is the common components shared by both in the system. The ignition system in it should be completely isolated from any of the boats wiring with the exception of the black and yellow stripe wire to the ignition switch. I would start by checking your wiring and see if any wires have been crossed as the tach being hooked into the ignition side of the stator could be your issue.