" Was hoping a post might give me some ideas on what to check on my 1992 120hp Johnson. I have it on an 18' Starcraft and while on plane running at ~80% throttle, the motor will slowly increase rpm and boat speed. Maybe like someone has increased the throttle position or its getting more gas. When I ease the throttle back enough to lower the rpm, its always too much and goes off plane. There does not seem to be a sweet spot for running less than full throttle.
I have replaced the fuel filter and recently tuned it up but this condition seems to be getting worse over time.