Just bought the boat in may. Maybe has 10 hrs on it. Everything works like it should except the tach acts goofy. I'll make a run and it seems like it's where it should be. Then next run it'll peg out past 7k. Motor never sounds any different or anything.  I'm assuming just a cheap tach and plan to replace it with the merc monitor soon. Anyone experienced this or have any advice? It's a 115 pro xs 4 stroke.  Prop and everything seemed right from the factory.  It would top out at 6k and run 49 mph first several trips 

Outside of a bad or corroded wire, I’d look at a bad rectifier, voltage regulator or stator.

Just talked to the dealer and they said check connectors and if that's not it it's a bad tach. Said the motor would let me know if I was over revving it

Get under the cockpit with a flash light and look at the back of the tach and there might be a tiny, usually black, plastic screw head sticking out with a flat blade slot in it.  With a screw driver twist it back and forth a few times (and remember where you started) and it might fix the problem.  

Take off the each 2 termibal connections one at a time st the tach and spray them with DeOxit to clean it and reinstall. 

If you know which wire by color code you can also remove the connections at the engine clean with DeOxit and retighten.

The screw mentioned is a adjustment for calibration, shouldn't affect it's overall performance.

Tom

I'll try those things. Thanks a bunch 

Yes sir, I know it is for calibration, but excercising the calibration screw on mine did stop my tac from wondering around and kept it on RPM.  Another boater advised me to try it