20hp 4 stroke can't run wide open

[MAisbett]

Hey,
I've got a 20hp 4-stroke I bought new in 2010.
I'm sailing around the world right now and use the engine A LOT for diving and exploring around.
The problem I'm having is that when I run at full speed, the engine gets starved for fuel. If I give a pump on the fuel line bulb, it keeps going...
I checked the fuel tank and found some kind of a growth on the bottom (I'm in the tropics) so I cleaned that out, cleaned the fuel filter, took the carb off, blew out the jets and generally gave it a clean up.
It's still having the same problem.
Anyone out there know what's going on?
Also, does anyone have a shop manual or know where I can get one for this engine? PDF possibly?
Thanks in advance,
Mark Aisbett
SV Merkava
Darwin, Australia

[99yam40]

see if you can get your hands on a gauge to measure fuel pressure/vacuum.
It should show if there is a restriction and if pump is working properly
A piece of clear tubing installed on the output of pump will help see if there is air being sucked into the system

[boscoe99]

If you can keep the motor running normally by pumping the primer ball that is a sure sign of a fuel flow issue to the carburetor.

Maybe the fuel pump needs to be serviced or there is a restriction in the fuel supply from the tank to the pump, which the pump cannot overcome.

Run a hose from a different tank directly to the inlet of the fuel filter. See if the problem goes away. If so, it is something upstream from the fuel filter. Could be a hose, the primer ball, hose quick disconnect fittings or something within the tank itself. If the problem persists, I would then inspect the pump.

Of course you have already changed the fuel filter. And verified that it is not leaking. I hope.

Yamaha does not offer service manuals in pdf. Hardcopy only. The parts catalog is available online however.

[fishwalton]

air

I had the same problem with a 2006 25hp Yamaha 2 cyl. and it turned out to be air getting into the fuel line. The female connector on the fuel tank was loose. Tightened up the connector and problem solved