2016 Honda BF90 HP Tachometer issue

[Landpanther]

My Tachometer suddenly started registering higher RPM but the engine is not revving up. Tach shows 3500-4000 RPM but boat is traveling about 5 MPH. When I increase throttle slightly, the Tach revs past 6000 RPM but speed is only about10 MPH. Is this a bad Tach, or does the Tach need adjusting somehow ? Any suggestions would be helpful.
(Engine run in Fresh water only , Engine has 175 hours, Regular annual maintenance performed)
Thanks.

 

[ahicks]

If it was working correctly up to this point, I would confirm it's getting 12 -13 volts, and the ground wire is making a good connection. That being the case, I doubt any adjustments have changed by themselves, so you may need a new tach.