"first question is about my tachometer. how do i fix it or check if prob is coming from engine. my boat is a 1986 bumble bee. i got a 1979 mercury 150 on it now...this is not the original engine of course but the tach has not worked since ive owned it. i think that another brand engine (unknown kind) came on boat so the tach. on the dash is whats hooked up to my merc. well the tach. sometimes bounces up and down like a broke speedometer in a car and sometimes it dont do anything. someone once told me to check the rectefier to see if its putting out right but how do i do that? second if it is putting out right, do i need to get a tach that compatible with a merc or is that how it works with marine engines."