1977 55hp Evinrude

Got an issue with dad's 55 Evinrude on his pontoon. It runs pretty good up till about half throttle then starts to surge after that. He noticed that when the engine starts to bog down a huge gush of water comes out the back of the engine. When it's not bogging, the water output is normal. What would cause the big gush of water when the engine goes to bog down? Is this a thermostat issue? Maybe temp sensor telling the power pack the motor is hot and it starts to limit the rpm then changes it's mind and decides it's not hot? Does a '77 model motor have the SLOW system on it?

No SLOW on this motor. Has anyone been working on his motor? Such as new water pump.

yep...I put a new impeller in it a few months ago

I was thinking maybe the drive shaft seal in the waterpump housing was not installed right and water could be pumped up into the lower motor cover, and flooding out the bottom carb.