My 90HP Yamaha 2 stroke loses water pressure as my RPM's increase. Once I get past about 4500 RPM the water pressure will drop to 0 or below on the gauge. Jack plate height does not seem to matter. Water depth does not matter, 8 inches or 8 feet, though lower water temps does seem to increase pressure. At lower RPM water pressure is from 5 to about 18 pounds. I've replaced water pump and that did help overall pressure but still cam not run wide open without pressure falling to zero. There is a cav plate made by Tran on the motor. What can I try to fix this problem? Thanks.

Does it do the same thing on a cuff (water hose) I would have the head gasket checked if it does. If it does it only when moving on the water, something is disturbing the water to the water pickup of the lower unit. You might check the poppet valve. Just do`nt see how poppet would cause it to lose all pressure.

Here's one that we found on a couple of motors. The water intake strainer had been put in backwards. When setting still, all was ok, as the motor traveled faster and faster the water pressure was less and less.........

Very good tip Mike.

How is your sled rigged? Do you have a compression plate? If not then you need one badly.

Thanks for the tips everyone. Pat, there is a cavitation plate or hydrofoil on the lower unit that Tran installed. I'm assuming thats what you mean by compression plate.

I've always thought that maybe there wasn't "clean" water getting to the motor at higher speeds.