Awesome fix! The tach on my '05 Mercruiser 4.3 was doing the same thing, showing about 2000RPM at idle, then it'd jump around erratically on higher throttle. So I followed your suggestion, turned that adjustment screw/switch back & forth a few times & now it seems to be working correctly again, about 800 @ idle then smoothly progresses up as the motor is throttled up, except . . . when the motor is turned off, the needle then sits on 1000RPM, but turn on the key again (not the motor, just the power) & it goes back to zero. Not sure how to fix that, but I can live with it. Be careful, though, when turning that screw/switch b/c on mine (Teleflex) there's no "click" or other indication as you move it; it just moves smoothly around. So . . . word to the wise: make sure you note the position of the screw/switch prior to adjusting it in order to get it back to the same place (assuming it's correct to start with).
