anti siphon valve might be problem. I have had to remove a couple of them on different boat in order to get adequate fuel flow. You can push the ball and spring out with a screw driver, then test to see if that solves the problem. If not, then I would replace it with another one (not expensive) as they are a nice thing to have (required
).
I have had 2 strokes that just pulled gas too fast (drank gas) and would close of the valve. Also, a newer yamaha 4 stroke has a low pressure fuel pump that turns on intermittently at low RPMs (becasue it doesn't pull enough gas at idle), when that pump woudl kick on it woudl lock up the anti siphon valve
