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  1. #1
    WARRIORMAN
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    Question on anti-syphon valve in fuel system

    What is the purpose of an anti-syphon valve and how does it work in conjunction with the fuel system? I have mallory electric fuel pumps with regulators feeding the carburator. Occasionally, one fuel line sucks air, and you can see the guage needle bouncing. Could this be a cause of the anti-syphoning valve not functioning properly, or an intank fuel line pick-up broken off?

  2. #2
    Platinum Member Platinum Member wildthing357's Avatar
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    I had a problem with the pickups in the tank, their was a small crack at hte top of the pick up tubes that the eye could not see.
    I replaced them and did knock out the check vavle on the anti syphon at the same time. This was recomended by the shop I bought the pickup tubes from.

  3. #3
    blown1500
    Guest
    Could you be talking about the anti drain back valve? Most boats have a ball type check valve at the tank to keep the fuel from running out of the fuel lines and pump after shut down. These can be very restrictive and can cause problems with the electric fuel pumps which don't pull very well. Recommend you check your fuel lines, etc. for any looseness or cracking first. I usually remove the check valves, but I don't recommend you do the same without your own research!
    Last edited by blown1500; 02-02-2003 at 12:42 PM.

  4. #4
    Registered Roger 1's Avatar
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    Hi Warrior Man;
    I`m no ''expert'' on the subject but I believe the reason for the anti siphon valve is to prevent the fuel system from loosing it`s prime when fuel is not being drawn from that tank. Also it is there to prevent any debris that may have been filtered from the fuel from being siphoned back into the tank. In addition, if you break a fuel line below tank level, it will keep your bilge from filling with fuel! That could be your problem because they are somewhat restrictive. I think that if your fuel pickup tube was broken of you wouldnt be getting any fuel at all. What you may be experiencing is ''fuel slosh''. You know, fuel sloshing away from the pickup tube. Also, Make sure that you don`t have a gauge problem. If it`s the gauge, and they do go bad, you can spend a lot of time chasing your tail....... Roger 1

  5. #5
    Registered Crazyhorse's Avatar
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    The anti-siphon valve is to prevent fuel in the tank from siphoning into the bilge or engine compartment in case of a fuel line rupture. This valve is essentially a ball with a spring in front of it so that the fuel pump suction must pull the check ball off it's seat by overcoming the spring pressure. These valves have been known to cause flow problems on systems that have a high fuel flow rate. If your fuel tank has multiple pickups you can utilize the extra pickup to provide the flow rate your system needs.

  6. #6
    Rick#29
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    Re: Question on anti-syphon valve in fuel system

    Originally posted by WARRIORMAN
    What is the purpose of an anti-syphon valve and how does it work in conjunction with the fuel system? I have mallory electric fuel pumps with regulators feeding the carburator. Occasionally, one fuel line sucks air, and you can see the guage needle bouncing. Could this be a cause of the anti-syphoning valve not functioning properly, or an intank fuel line pick-up broken off?
    Question 1 do you have 3/8ths or 1/2 inch lines, if 3/8th remove the ball check see if this fix the problem .If it did the pickups are to small for your engine program. If you have 1/2 inch lines and are having problems change the check valve ,that check valve is big enought for that mallory pump.

  7. #7
    Registered Crazyhorse's Avatar
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    yup, what Rick said.

  8. #8
    WARRIORMAN
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    Thanks for the replies guys. It is at Bobby Saccenti's right now. They are going to go through it. I have the 1/2" lines with the Mallory 140 marine pumps. Thanks again.
    Little Tommy

  9. #9
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    Anti-Siphon Basics

    The anti-siphon valve, at the tank suction fitting, is to prevent fuel from siphoning out of the tank if there is a break in a fuel line that is BELOW the suction fitting and below the fuel level in the tank. Fuel cannot siphon uphill unless it also has a solid line filled with gas that is below the fuel level. If none of your fuel lines or any fuel components are below any possible fuel level in the tank, you donít need an anti-siphon valve. It was probably made necessary by the Coast Guard as a catch-all for every fuel system installation, without regard to your specific system design. Thatís one of the many reasons government regulation is very often a bad thing. However, because the rule is there, you might have to convince your insurance company that it was never necessary if your boat burns. The anti-siphon valve can also be considered to be an anti-flow-back valve which could prevent fuel from just running back into your tank requiring a system prime and perhaps also carrying filtered debris out of your fuel/water separator with it. Every fuel pump pushes fuel better than it pulls fuel. A good solution to the flow back problem is to install a check valve on the discharge side of your fuel pump.

  10. #10
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    Quote Originally Posted by bbezenek View Post
    The anti-siphon valve, at the tank suction fitting, is to prevent fuel from siphoning out of the tank if there is a break in a fuel line that is BELOW the suction fitting and below the fuel level in the tank. Fuel cannot siphon uphill unless it also has a solid line filled with gas that is below the fuel level. If none of your fuel lines or any fuel components are below any possible fuel level in the tank, you donít need an anti-siphon valve. It was probably made necessary by the Coast Guard as a catch-all for every fuel system installation, without regard to your specific system design. Thatís one of the many reasons government regulation is very often a bad thing. However, because the rule is there, you might have to convince your insurance company that it was never necessary if your boat burns. The anti-siphon valve can also be considered to be an anti-flow-back valve which could prevent fuel from just running back into your tank requiring a system prime and perhaps also carrying filtered debris out of your fuel/water separator with it. Every fuel pump pushes fuel better than it pulls fuel. A good solution to the flow back problem is to install a check valve on the discharge side of your fuel pump.
    9 year old thread revival???


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