[bepersistent]

Hi, my Merc 115 4-stroke recently developed a problem where the tach readings jump up and down several hundred RPM, the throttle wasn't moved. Eventually it evens out and the WOT rpm is as advertised. All I've done in recent months was replaced the spark plugs.

[JUSTINTIME]

clean the connections on the back of the tach
might be a bad connection at the rectifier or a bad rectifier

how many volts are you getting at WOT