120hp Johnson increases speed while on plane

[mtd]

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.

Any advise?

Thanks,
mtd

[Bob Z]

If you do not get a response here for to http://www.marineengine.com/discus/

There is a guy on that board named Joe and he will likely have an idea for you.

[CarpetBagger]

I think that has something to do with the pitch of your prop... once its jumps you up on plane it has a tendency to want to accelerate.

Has it always done this? Or maybe have yopu ever changed you props?

CB

[markman]

If it isn't the prop check the throtle line tension. It may be a little loose and moves with the vibrations to a certain point without actually moving the throtle. Then when you pull off a little bit, it jumps.

[Osprey]

I'd try a different prop just to see what happens. 80% throttle seems like a little much to plane your boat out. Try a lower pitch by one size or maybe a 4 blade. Most prop shops or dealers will have different props you can borrow. It's an easy thing to check before getting into paying someone to look at your engine.

"Osprey"