78 Mercury 900 won't run at full throttle

[Fultzrods]

Clean fuel, new fuel filter, carbs cleaned, still won't stay throttled up under a load. Starts and idles fine. Even runs at slow speeds just fine. As soon as it gets on plane though it throttles down and dies but will start right back up and idle fine. This is on an older 90 hp Mercury 6 cylinder outboard motor.

 

[Chris1956]

Have someone pump the fuel line primer bulb. If that fixes it, rebuild the fuel pump. If the bulb collapses, you have a restriction between the primer and the tank, or the tank breather is clogged,

 

[444]

Agreed. IMO any old engine should have a new pump kit installed. They are cheap and easy to get out of the way. Over time with age the diaphragm can develop small holes or leaks, get hard or the seals can let go. Enough that part throttle may be okay but not enough juice for full throttle.