We just purchased a 1994 pontoon with a 40hp 4 cyl Merc Tracker. The tach is acting up...while idling the engine the tach may read 1000rpm and then it will bump up to 1300 to 1500 and has bounced around from 2000 to 4000...all while idling. Wide open I have seen numbers ranging from 5500 too 6500. I really want to trust this tach so we can dial in the propeller (I think I can add another couple inches pitch). Does anyone have an idea why this would act up like this? All electrical on the boat seems fine, motor always cranks and starts easily, never misses a beat. Volt meter shows proper charge rates at various throttle settings. What do you think?