20HP loading up under power

[rcoogler]

I have a Yamaha F20PLH, Mfg 12/06, 20hp 4 stroke power tilt/elec start. After sitting for a few months, the motor starts right up and idles, even goes to about 1/4 throttle, but then loads up and looses power. Motor has maybe 30 hours on it..just like new. Has been through one service. Carb was removed and cleaned by an auto-mechanic friend of mine. No dirt or gum or plugged jets were found. Replaced carb and had the same problem. Plugs now have been replaced...same problem. Fuel is clean and new, hoses are the same age as the motor and primes well. Tank vents are working. Any ideas?

[boats.net]

Try pumping fuel primer bulb when it looses rpm's, if makes it run better then starving for fuel, if makes it run worse then carbs are flooding.