15 hp 4 stroke slipping

[DeadShot]

I've got a 2007 15 hp 4 stroke Merc that seems to be slipping. The engine RPM's will go up and the boat will slow down, then it will catch and pick up speed again after a while. If you let off the throttle it will catch again also. First though it could be cavitating but thats not it. Though it was the prop slipping but bought a new prop and still the same thing. Anyone got any ideas??

 

[JB]

Re: 15 hp 4 stroke slipping

First though it could be cavitating but thats not it. Anyone got any ideas??

I think you mean ventilating. What evidence did you use to decide this was not your trouble??

Where is the antiventilation plate relative to your boat's bottom?? Even, above (how much) or below (how much)??

What is the pitch of your prop? What rpm is the engine turning when this "slipping" starts??

 

[DeadShot]

Re: 15 hp 4 stroke slipping

It is even with the bottom of the boat or just a hair below, and it was happening just at WOT but now it seems like it happens when you give it full throttle all at once it but not if you slowly bring it up to speed. This motor has been on the same boat since it was brand new back in 07 and this just started happening about 4 or 5 months ago, so I don't think it could be "ventilating".

 

[JB]

Re: 15 hp 4 stroke slipping

Well, it seems that your engine is correctly mounted, so that should eliminate ventilation. New prop should eliminate cavitation and spun hub.

What is left?

Could something on/in the bottom of the boat be creating a bubble stream for the prop to ventilate in?

Something in the drive train must be slipping. I guess the way to find that is to tear down the leg, but maybe a Merc-spert has a better idea.

Good luck.