anti-siphon valve???

[Ross]

Any know know who sells anti-siphon valve with 3/8 pipe to 1/2 tube.New regs requires my 1964 fuel system to have them.I really dont want to change to rubber lines.Thanks for any input.

 

[MikeP]

You should be able to get fittings at any plumbing shop to easily adapt it to what you need. Every A/S valve I have seen needed some sort of adapter fitting to fit the tank/line.

I'm NOT suggesting that you shouldn't put anti-syphons on, but I didn't realize regs REQUIRED retrofitting anything on pleasure boats that were manufactured before the requirement existed.

The anti siphon requirement - last I paid attention was 5-6 years ago when we had a gasser - was that IF the carburetor is ABOVE the top of the fuel tank, an anti siphon valve is not required. If it's below, an antisiphon valve IS required.

 

[smoothmove]

While we're on the subject, can someone give a quick explanation on how these anti-syphon valves work? I remember on some old Chris Craft woodies I had, the anti-syphon function was performed by a small hole drilled in the top of the riser tube in the gas tank. The engines slurped up enough gas to overcome any air in the line. Reason I wondered about this, is that my generator will start and run happily for about 10 or 15 minutes and then stop, with a dry carb. I'm wondering if the generator is sipping the gas so lightly that the anti-syphon valve "thinks" it's a syphon leak and is introducing air to break the syphon, thereby stalling the generator.

Eric

moved from incorrect thread

 

[yachtsmanbill]

Moved this for the benefit of continuity also...

Default Re: Gasket on genset access hatch on 36C

The Onan on my Roamer has its own pick up tube, only down to about 1/3 level so you dont inadvertently run the tank dry. I had a major fuel supply problem when I repowered, starving for fuel above 3000 rpm. After A LOT OF HEAD SCRATCHING for almost two years, I surmised that the anti syphon hole was causing this. I soldered the hole closed and installed Searay syphon checks (gulp). The fuel pump vacuum has to overcome the spring in the check valve to pull fuel through. That wasnt the problem. The culprit was the braided copper flex line to the engine sucking air. If the syphon break wasnt there...
The way CC ran the fuel lines from the tank top to the bilge calls for anti syphon protection. Even if I had a 1931 Wheeler, I would install the device. You can not risk raw fuel in the bilge from a cracked line!!
My generator line runs UP to the deck joists and over to the generator's flex line. The whole fuel system is above the fuel level in the tank. No way for a syphon to start. ws

 

[MikeP]

Typically they are just a spring loaded ball against a seat. The suction of the fuel pump opens the valve (pulls the ball) away from the seat against the spring but the syphon suction alone (fuel pump/engine off) is insufficient to overcome the spring pressure on the ball. They can clog at the ball and restrict fuel flow as others have pointed out. This happened to me as well on our previous boat (Crusaders).

Without an anti-syphon valve, if the carb is below the fuel level in the tank, the only thing keeping the gas from filling the carb, intake manifold, cylinders, bilges, etc is the needle valve in the carburetor. EEK!

 

[Ross]

coasty regs i beleive state that if fuel line is below the tank anti-siphon is needed.Not many times dose the line run up hill all the way to fuel pump.