strange topping out

[Jack h]

my club rib is a 5.3m aircraft rib with a a 4 year old yamaha 50 2/stroke.
it tops out at about 4,300 revs at WOT but then backs down quite quickly to 3,900 then creeps up again to 4,300, this continues unless you cut the throttle back a bit.
The out of the hole performance is good but the top speed is quite poor.
I would have thought it was overpropped heavily but due to the way the engine 'hits a wall and backs off' i'm wondering if there is a porblem,
but when we first got the rib it was great and did 5,600 WOT.

It's been holding back as it has done for most of it's lifetime, just wondering if there is an easy fix?

[Hightower]

If hole shot is good then it's not the prop as a good holeshot will mean higher revs. Sounds like a problem with the over rev limiter. That'll drop your revs quickly to a "safe" level.

[Polwart]

Fuel supply problem? Perhaps blocked filter or carbs? Or even tank breather...

[Jack h]

thanks for the help sp far, the first answer sounds more likely, anyone else got an idea?