Motor revs higher, speed down

[Eyeyiyi]

18’ pontoon Bf50a Honda after pulling it out and rebuilding carbs and doing fluid changes and new water pump I just got her back in the water motor ran great revs to 5500 at 16 mph. I bought the boat used and it used to go faster . The prop is originally and beat up so I got another alum prop same pitch 11.4x12 installed I only get 5000 rm but gained 2 mph which find unusual but acceptable. My problem is with the new prop after 5 to 10 min at wot all of a sudden the revs jump and boat slows. Checked to see if hub spun nope marks still aligned . Is there something else that I didn’t get aligned right when I did the water pump ?

 

[jgmo]

Wierd.

I don't know but is your transom in solid shape?

I can't think of anything you could have gotten wrong putting the gearcase back on that would cause this.

Just a wild guess really...
....but if the transom is letting the engine move around that could set up a cavitation situation.

18 mph sounds slow for a 50 hp pushing a 18' pontoon to me. So, I would definitely be looking at how the outboard is mounted even if the transom is rock solid.

Any chance your toons are taking on water?

I've never run across this before so just tossing out some thoughts.

Good luck.