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.
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.
Safety
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.