40 hp 2 stroke pees fine at low sped, quites at high speed

I've got a 2004 40 hp 2 stroke that I just did some work on. Since i had the foot off i replaced the waterpump impeller. Old unit was in good shape.

Thing runs great and pees like a 3 year old untill it gets to around 3000 rpm, then quites and starts to overheat.

Slow it back down and it starts peeing again.

I's pretty darn sure the impeller is proberly placed on the key. Can the thermostat cause this? Didn't mess with that part.

Thanks for any suggestions.

Check the water intake on the foot.

Thanks, I had done that. I can't figure this thing out.

I assume that it worked fine until you changed the impeller. the fact that it overheats, certainly would point to it not getting water. I wonder if you installed the wrong impeller, one with a larger inner diameter that may be slipping on the shaft at higher rpm.

Thanks, but pretty sure it was right. Tight fit on shaft, was carefull to get key lined up right, original Yamaha part.
Getting ready to pull it back off (as soon as i can get the boat back on land), don't know what else to do. Just weird the way it works fine untill reved up high.

not sure if your engine has one but look to see if it has a poppet (pressure) relief valve on the motor.
they can cause idle and or high speed overheat problems.

at idle speed - if the valve opens (should stay shut)- it will bypass some of the water going thru the cylinders - which won't allow proper amount of water to reach upper cylinders and prevent cooling = overheat
at high speed - if valve won't open - the water pump will just create more pressure (absorb more heat but can't exchange the water) instead of flowing faster because some of the water pressure won't be relieved thru the poppet = overheat

better explanation:

The poppet valve regulates water pressure in the block. At idle your water pump barely pushes water through the block, especially through the top cylinders. When you increase rpm's the water is forced through the motor by a combination of the water pump and the force of the water pushing into your water pump from forward motion in the boat. The poppet valve will open when the water pressure in the block becomes too high and it relieves the motor of this extra water pressure by redirecting the water out the exhaust ports. If the valve is too weak or has a leak it let's water pass that is needed in the upper parts of the block.

Thanks, I'll check and see, but i don't remember seeing it in any of the breakouts. Would it be in the power head or the lower unit?

Don't know if anyone is still following this thread, but was it ever resolved? I am having the same exact issue