Honda 25hp 4 stroke wont go above 4000rpm

[surfingcobie]

Hi
I have a honda 25hp 4 stroke that whilst in the water will not go above 4000rpm and we are stuck at 8mph (sat nav) .it's on a pilot dory 15ft boat . It revs all the way to 6000+ with muffs on out the water I've tried trimming it whilst going but no joy
Someone mentioned propeller pitch mine has these numbers on it 992502 10p anyone have any ideas ??

Many thanks cobie

 

[NSDON]

Is this boat and motor new to you- did it go faster before and now doesn’t?

 

[surfingcobie]

Is this boat and motor new to you- did it go faster before and now doesn’t?

Yes it's new to me and the owner before me never used it as he had another boat to use ,so it did sit for a year but it ticks over lovely and I've put fresh fuel in

 

[jgmo]

What you describe is a "no load vs. loaded" condition.
IOW....it revs ok in neutral but doesn't want to do all the work.

That says....at least to me....that you may be down a cylinder.

These engines can run surprisingly smooth with a dead cylinder and fool you.

I recommend doing a cylinder contribution test to prove/disprove my theory.

First though you might just remove and inspect the spark plugs. I would replace them but that's your call. Make SURE that you use/smear dielectric (tune-up) grease inside the plug wire boots before reinstalling them.

 

[richard.b.humphrey@outloo]

When you say boat was not used for a year was that in or out of the water if it was in the water hull may need a clean.