engine overspeeds

[dilbert88]

I have a 2004 yamaha 115 hp 4 stroke with 290 hours on it. It acts like I have a bad prop hub. I replaced the prop with my spare and it still does it. I bought a new prop and it still acts like it is slipping. Close to 6000rpm at 15 mph. I use to be able to do 35 mph easily.

[robert graham]

With direct drive from the powerhead to the lower unit/gearbox, about the only thing that can "slip" would be the prop hub. Check your lower unit oil for contamination/water/metal particles....could be a problem in the lower unit.

[dilbert88]

Thanks Robert, I am guessing that what the book calls clutch dogs does not have any plates that will slip and if the dogs were slipping, I would hear them clicking?

[99yam40]

Yes just like jumping out of gear

Are you sure you have not changed up anything else on motor mounting or boat that would cause prop to suck air?

[dilbert88]

My boat has been out of the water for a year and when I put it back in the injectors were fouled. Could not get over 3500rpm. Cleaned the injectors and it now goes up to 6000rpm. When I pulled it out a year ago it had growth and barnacles that I scraped off with a dry wall knife. The surface is smooth but not like original. I did not want to sand it fearing to mess up the Gel coat. No engine geometry has been changed. It does seem like it is cavitating though. Your input is appreciated.

[dilbert88]

I found the problem. The depth transducer had rotated a little and slipped down just below the plain of the bottom of the boat. It was disturbing the water in front of the drive and causing the prop to cavitate. The boat is working well again. Thanks for your inputs..