is this in water or on trailer? Without any load the its easy to hit the limiter but there's a limiter for reason. But in the water and it does that sounds like a worn jet pump. Wear ring could be worn and impeller could be damaged causing cavitation.
A visual inspection will work, make sure there's nothing obvious wrong with it like a chunk missing out the impeller, check the gap too. were the impeller meets the wear ring. Any large amount of sunlight missing it'd be a safe bet to replace the wear ring and do a service on the jut pump.
Last time you serviced the fuel system? Original oem fuel lines (gray in color)? Used before ethanol was introduced and overtime it will create a green goo like substance clogging your carbs.
What are the condition of the plugs too.