2005 F40 Fuel Delivery Troubleshooting

[Rebelyelp]

Last year, I rebuilt the carbs on my 2005 Yamaha F40. Once I got them tuned, etc. the motor ran perfectly.

Fast forward. Motor cranks and idles fine (outside of a pesky solenoid issue). Motor goes WOT, boat gets up on plane fine, then begins to miss like it's losing fuel. I can pull it back to idle for a few seconds and bury it again, only for the process to repeat. My belief is that the carburetor bowls are filling, but not refilling rapidly enough to replace the burned fuel. Is there any way to troubleshoot between fuel line, fuel filter, fuel pump, etc. without replacing everything? Any other possible causes ? I am going to replace the fuel filter anyway as it is probably due, I don't have an additional fuel line to test against it.

[walleye1]

Check your fuel pump per service manual with a mityvac.

[panasonic]

As Wally said, it's sounds like your fuel pump is not keeping up with the demand at WOT. You could also check that your fuel tank is venting properly... make sure primer ball is not collapsing when at WOT.

[Rebelyelp]

As Wally said, it's sounds like your fuel pump is not keeping up with the demand at WOT. You could also check that your fuel tank is venting properly... make sure primer ball is not collapsing when at WOT.

Yes, checked the tank vent first, and bulb. Meant to add earlier, with a passenger slowly squeezing the bulb, I have no issues. Again, led to believe it's the fuel pump, just wanted to dot my i's and cross my t's before I order one. I'll see if I can locate a mighty vac/service manual. Thanks guys!

[rodbolt17]

wow I wonder how long that rubber diaphragm can last.
that pump is easlily rebuilt.

[Swimmer451]

I have the exact same problem on my f40...2001. #2 cyl is dropping out at higher speeds. I suspect it is my fuel delivery system. It runs better when I squeeze the primer bulb. When I bought it, it was doing this and then ran fine for the summer. Now it is back. I’m headed to rebuild the fuel pump. I hope that is it. I’ve had the carbs apart enough.

[Swimmer451]

One of your floats is probably closed. When you pump the bulb it forces the float needles to open enough to run for a bit. If you have it apart you might find one float needle is longer that the others. They slip out and prevent the float to cycle or float. That happened to me. Took a bit to figure it out but it runs better now that I git new ones.