Stock 440 cavitating

[NBraun]

I have a mostly stock 87 440 that i put back together and got running over the winter. It runs fine till about 50% throttle, but if i go over that or full throttle, the ski will just rev up and won't give any thrust. From what i've read its because it doesn't have a good seal around the pump. So what i'm asking is, does this sound right, and if so, what exactly do i have to seal up, and with what?

Is it at the front, where those pieces meet the sides of the intake?

 

[Fleabags]

You want to seal up the pump where it contacts the hull up inside the pump cavity. Also, seal your intake grate. Use 100% silicone or it will wash away. The goal is to stop the flow of water rushing thru an opening that will disrupt the flow of water thru the pump. Any flow entering the pump area that is not intended, will aerate the pump and cause some cavitation.
However, before you seal it all up, I'd take the grate off and measure the clearance between the impeller and the pump housing. If you have a lot of wear, and too much gap, you will cavitate.