Helping a friend get his duck boat ready and have run into a fuel problem.
 
While running down the lake ¾ throttle no problems... anything more than that it will suck the fuel bulb shut, which kills the motor. So far, he has replaced the fuel line, tank and it is still doing the same thing. If you remove the cap on either tank, you can run wide open all day without any problems.

any suggestions ?

+1

If it has portable tanks then you may need to crack the cap slightly ( I remember having to do that in the old days)