2015 9.9HP four stroke loses power under load

I need some help diagnosing issue. I have rebuilt carburetor and cleaned all the jets and ports that I know how too - twice. Cant see anything more I can do there. The motor runs great when hooked to a hose or running in a tub but when its in the water it will lose power. It will occasionally gain power and run to full throttle but then stutter back barely running. I will crank fine and idle to full, it does fine when in tub or on a hose, but when you put it under a full load it poops out. Any ideas? Fuel pump? fuel Hose?

They all " bark and roar " on a hose.-----Will do that running on 1 cylinder !----Means nothing.-----So do some checking.-----Compression test.----And does spark jump an actual gap of 5/16" ( 8mm ) or more ?

racerone said:

They all " bark and roar " on a hose.-----Will do that running on 1 cylinder !----Means nothing.-----So do some checking.-----Compression test.----And does spark jump an actual gap of 5/16" ( 8mm ) or more ?

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I will have to check the 5/16” gap. The motor may has no more than 20 hours on it. It sat for a few years Unused so I did replace the fuel, in line fuel filter, and spark plugs. Does that change anything?

Some will argue.-----It costs zero $ to do the tests suggested.-----If the motor sat for a few years with fuel in the carburetor the jets were likely plugged.----On more than one occasion we see folks struggling to clean these carburetors.----The passage ways are very small and easily overlooked.-----If the motor passes the 2 tests clean or have the carburetor cleaned by a shop.

The narrowest opening which feeds the fuel is the jet or jets. These may look open, but residue can cleverly close them up enough to create fuel starvation. They need to be cleaned "physically" to the correct size. I have a nice welding tip cleaner and after soaking down with a very strong cleaner ( like CMC Throttle Body Cleaner), the deposited material will surrender to your welding tip cleaner. Many folks will assume that if the jet is open and shows good passage with air or a carb cleaning agent, that all is "golden". That is often not the case and the carefully engineered flow through the jet, turns out to fall short of perfect. The old motors had adjustable jets so this "perfection" of tuning was in the hands of the operator. Things are "engineered" now to be user friendly and less opportunity to mess it up. We really have to make sure that jets are the exact orifice size, or performance as well as engine/motor life will be compromised.