Newbie- Problem with engine or boat?

[deekdeek]

New to this forum, would really appreciate some advice on my Yamaha outboard – not sure of the exact model number etc. I have a small cruiser and have bought a 2^nd hand 25HP Yamaha 4 stroke outboard. After fitting it, everything seemed fine, that is until the boat is actually underway. When you advance the throttle the revs increase until you hit about ¾ throttle. As you advance the throttle further, the engine revs decrease and the boat obviously slows down. The more throttle you give after the ¾ position the lower the engine revs. Bringing the throttle below the ¾ position, again, all seems fine. Looking through this forum, I thought it sounded like a fuel problem, so engine mechanic removed and cleaned carburettor in an ultrasonic bath, cleaned all fuel lines etc, etc. Alas problem is still the same. Mechanic is now saying the boat is under-powered with this engine and that is the problem. Logically I would think an under-powered boat would not travel as fast as a correctly powered one, but surely the engine revs should not decrease when you apply more throttle? Am I right in thinking there is a fault with this engine? I suppose my simple question is – is there a problem with this engine or a problem with my boat!! Advice greatly appreciated.

[brian3006]

what kinda boat is it on? I have seen some badly underpowered boats do some odd things at WOT

[deekdeek]

Boat is about 35 years old, no identifying plates etc are present. It is about 14 feet long, beam is about 5.5 feet and weight is around 600 pounds. It has been taken out of the water and the hull was thoroughly cleaned a few months ago.
I was told it would need about 40HP outboard to make it plane (with maybe 2 adults on board) , but at the moment I am not concerned about speed etc. I just want a slow cruise!

[DennisG01]

Well, crap, reading your first post where you said it's a "cruiser", I would have said VASTLY underpowered as a "cruiser" style boat is one with at least one sleeping berth in a cabin... certainly much, MUCH bigger. It might have been good to lead with the 14' part!

Defintiely sounds fuel or carb related. It's not boat related. Have you check your fuel/water separater? Checked fuel quality for water? How old is the fuel? How does one "clean" fuel lines? It's possibly for lines to fail internally, creating a check valve, of sorts.

Was the carb tuned/adjusted?

[99yam40]

try a different prop

[brian3006]

Yes I too was expecting a much larger boat. a 25 shouldn't be bad on that boat. I have a 15hp on my 14 foot boat. It is def a motor issue. as 99yam said, try a diferent prop. (probably smaller pitch) if no change it sounds like a fuel or possibly timing issue

[deekdeek]

Thanks for the advice. Will try the prop solution as suggested. The mechanic had the engine for a couple of days and he assured me carb had been cleaned and in his own words he "had cleaned all fuel lines".