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What is a Fuel Anti-Syphon Valve?

Boat Manufacturers are required by law to install an anti-syphon valve in the boats fuel system. The valve is usually a hose barb fitting with a checkball and spring inside. It is usually mounted to the fuel tanks pickup tube where the rubber fuel line attaches. Some anti-syphon valves are mounted to the fuel filter assembly on the outlet side.

The purpose of the anti-syphon valve is to prevent fuel from syphoning into the boats bilge in the event of a break in the fuel line.

The problem is that the checkball inside the valve gets worn and tends to jam thus restricting the flow of fuel.

Anti Syphon Valve

The symptoms of a bad anti-syphon valve are, hard starting, surging RPM's while under way, hesitation or backfiring upon acceleration and lack of wide open throttle (WOT) RPM's.

The solution is to replace the faulty anti-syphon valve.
That said, many customers elect to just knock out the ball and spring or replace the valve fitting with a standard hose barb. This can improve performance and is less troublsome HOWEVER the owner should be aware that he/she is modifying the manufacturers design and taking some risk. Fire or explosion can result from fuel leaking into the bilge. Although it is a rare failure, fuel lines can break or come loose and allow fuel to syphon into the bilge area thus causing damage, injury or death.

Be sure to use fuel resistant pipe thread sealant (Permatex 2B works well) to properly seal the threads of the fitting. Always check for leaks.

Lubes, greases and oils can be bad for your health. Use rubber gloves. Always wear safety glasses.
Fuel vapors can EXPLODE and cause damage, injury or death. Never start or run your engine when fuel vapors are present. Never cause sparks or work with voltage or open flames when fuel or fuel vapors are present. Clean up all spilt fuel and let the bilge area ventilate until no vapors can be detected. Be smart.
Change your Alpha Gear Lube
Change your Bravo Gear Lube
Why does my drive lube have white streaks?
Why does my drive lube have gray mush?
Replace Your Alpha One Water Pump
Replace your Alpha Gen II Water Pump
Rebuild your Bravo Raw Water Pump

Troubleshoot & Replace your Alpha Shift Cable
Troubleshoot & Replace your Bravo Shift Cable

Replace your Alpha One Bellows & Gimbal Bearing
Replace your Alpha Gen II Bellows & Gimbal bearing
Replace your Bravo Bellows & Gimbal Bearing
Why does my drive "Growl" when I turn?
Water is leaking into my boat from the transom

Replacing your Trim Sender Switches
My Trim doesn't work but my Trailer Switch does
My Trim Gauge is Pegged

Rebuild your Alpha One Upper Driveshaft Housing Assembly
Rebuild your Alpha One Lower Gear Housing Assembly

Check your Engine Alignment
Check for Excessive Steering Play
Troubleshoot an Overheating Problem
Should I remove my thermostat?
Why do I overheat when going fast?
Troubleshoot a No-Crank Condition
Troubleshoot a No-Start Condition
My Engine Backfires and/or Surges and has Lack of Power
What is an Anti-Syphon Valve?
My Engine blows Steam from the Exhaust
Why do I hear a "clakity" sound while at idle?
Exhaust Manifolds...IMPORTANT Information
How do I Winterize my engine?

What is Galvanic Corrosion?
I have a Bravo III Galvanic Corrosion Problem
What is a Mercathode?
All about Zinc Anodes
How do I check my carburetor?
What carburetor do I have? A Rochester or a Mercarb?
I have water in my fuel
My fuel smells like varnish
My fuel is old
My engine has been sitting for several months without being run.

I have a Mercruiser 470, 485, 488, 190, 3.7L Engine
I have coolant dripping from the front of my engine
Can I install a regular Alternator instead of the Stator-Rotor?

Converting an OMC to a Mercruiser

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