89 mercruiser 120hp fuel starvation

[s.hadley81]

Hey guys, last night while I was running about 3000rpm my boat kinda fell flat on its face. I backed off the throttle and then slowly eased back up on plane and it seemed fine so I anchored up at my fishing spot for a bit. On my way back to the slip it did the same thing but this time it stalled so I pulled my fuel/water separator and it made a hiss like it pulled in air. I got the boat fired back up but it ran really rough under load so I took my fuel cap off and still no better. I limped back to the dock but anytime I gave it any throttle above 1500rpm it would stumble and die. I'm going to go mess with it on Monday but my question is would a bad anti siphon valve cause me to pull a vacuum on the fuel system? I figured I kinda ruled out a clogged vent by running with fuel cap off, I also swapped my ign coil out just cause I had one with me, I checked the sight tube on my fuel pump and I had my carb off and cleaned it along with my filters about a month ago to clean up an idle issue. i figured it's starvation and not flooding because it would fire right back up and it's fine in neutral it's only under load. Any input is much appreciated!

 

[Bt Doctur]

anti-syphon valve?

 

[s.hadley81]

anti-syphon valve?

The Anti siphon valve on the fuel tank. Sorry just to make my question a little clearer, would a stuck/bad anti siphon valve draw a vacuum on the fuel system so that when I cracked my fuel/water separator I would hear air rush in. I've never heard air rush in before when I took my filter off. I just never had one fail one me so I'm not sure how they act when they do fail. I use the boat about 3 times a week so I doubt it's a clog due to debris be cause it just happened outta the blue. I figured the first thing I would do is run on a remote tank and go from there.

 

[Bt Doctur]

The fuel pump is sucking ,if theres no fuel it will suck air from the fuel line.that creates a vac.
like sucking thru a straw with the other end plugged with your finger

 

[s.hadley81]

Thanks for the reply, I'm gonna look at my pickup tube as well to see if it has a crack or pin hole in it. That and the valve are about the only things I haven't repaired or replaced on the boat.

 

[s.hadley81]

I got it straightened out, It wound being a bad anti-siphon valve. I figured I would post to close out the thread. Thanks a lot.