Losing power

Have a new 50 HP Mercury 2 str

Have a new 50 HP Mercury 2 stroke engine on a pontoon.Starts ok and runs fine at low rpm.When rpm is increased between 40-45 engine starts to REV UP. rpm gauge then goes up to 50-60 range even though the engine seems to be slowing down. Mechanic has been here 3 times but the problem is intermittent and never happens when he's here. He has checked the fuel line and replaced the primer bulb. Some air was getting in. We took the boat out 1 hour yesterday and it was fine.I took it out after he left with a friend and ten minutes into the ride it started doing its thing.Not once but all the way home.Had to keep it at low rpm to get home(below 30)He's about had it.Any ideas?? HElp??

"One of two things is happenin

"One of two things is happening - the prop is breaking the surface of the water (losing traction) or the prop itself is bad (yes even if it's new).

The prop hub could be spinning (slipping on it's rubber sleeve).

The prop is made of two components - the hub is right in the middle and the fins are on a ring that is pressed onto the hub (which is surrounded by rubber). It's built like that so that if you hit bottom, the rubber will give and hopefully you won't chew up your fins too bad and save the gears from getting destroyed.

Any chance somebody grounded the boat, even on a sandy bottom getting in/out from the dock?

The way to check is place ajacent marks on the hub and on the prop itself and take her out. When it "does it's thing", stop and check the marks - if they are NOT lined up, you have a spun hub...

If the motor is simply breaking the surface, you have a rigging issue (motor is mounted wrong, not trimmed properly or have too much weight in the front of the boat when the problem occurs)."