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Proplems with anti-siphon valves restricting flow?

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  • Proplems with anti-siphon valves restricting flow?

    Has anyone had experience with this?

    This winter I installed a new fuel system. The tank manufacturer ships the tank with an anti-siphon valves installed in the pick-up tube fittings. Now my Johnson/Susuki DF 140s do not pick up at fuel after it reaches 1/2 tank level. I suspect it is because of the valves. All new hoses, filters, water separators installed at the same time. Pick-up to engine distances are shorter than before, the lifts less as well, the vent is clear, and the bulbs have fuel in them when the failure occurs.

    After refilling the tank, engine performace is fully restored. Ran 20 minutes at 4,700 RPMs without any fuel starving, then shut down to idle and resumed normally maneuvering. There are no restrictions in the lines other than the anti-siphon valves, so they will be replaced with straight through barb fittings.

    Has anyone encountered this?
    Last edited by flounder; 07-05-2020, 10:43 AM.
    Ole Joe
    Just floundering around
    White Hall & Ocean City, Maryland
    1978 Mako 25

  • #2
    If it pulls fuel when full but not when half full, I doubt it is the anti-siphon valves. The fuel level may be lower than the pumps can lift though. It could be bad low pressure pumps but it could also be an air leak in the fuel system. Check the primer bulb to see if pumping it gets it firm and it stays firm for a while. Check the seating of the on-motor fuel filter bowl. Check the fuel lines for air leaks, and check the water separator for a bad seal. It is possible I suppose that the wrong length pickup tubes were installed. You should be able to pull one, measure the tank depth and compare the pickup tube length to it.
    The fuel pressure test mentioned is a good idea also.

    Comment


    • #3
      Where is the tank pick up tube?
      Front or back of the tank?
      I had an Aquasport that the fuel tank was replaced. Trouble was it was installed backward with the pick up in the front. Ran out of gas on a plane with 100 gallon gas tank with 40 gallons in it. Slow down and everything was good.

      Comment


      • #4
        Both pick ups are in the rear. The fill and vent are both up front. From the rear of the tank there are short runs to The Racor filters plus short runs from there to the engines.

        I have removed both anti siphon valves since posting. Have 3/4 full tank currently and am running fuel level down a little at a time just fishing in the back bay. Will see how she does when we get to 1/2 full. If they fail to pick up fuel below that level, will try to pull the pick up tubes.

        Others have overcome the problem by removing the AS valves.
        Ole Joe
        Just floundering around
        White Hall & Ocean City, Maryland
        1978 Mako 25

        Comment


        • #5
          It seems to me that if there were a flow issue, the motor wold start to starve and then stutter. Lowering the throttle should result in the engine running OK. If, in your case, the symptom is that the engines dies at 1/2 tank and doesn't run until you add gas, I would expect that to mean that the pickup is not getting gas ( level too low).
          The fact that the bulbs have fuel in them is confusing. If you pump the primers, does the motor start and run for a while? I still suspect an air leak or short pickups. Let us know what happens.

          Comment


          • #6
            Motherboat,

            Yes, could be short pick up tubes...or perforation of the tubes 1/2 way down, but how likely is it that. The manufacturer says "it would not be the first time", so I guess we can't rule it out.

            However, considering the primer bulbs are still full, I doubt it's an air leak up-stream. They continue to pick up more fuel when pumped which indicates the tubes still reach the fuel. And, after prime, they idle fine till prime is exhausted with the bulbs still full. The bulbs remain firm with the engines shut down. They do not lose fuel when un-attended for several days so I'm certain the check valves in the bulbs are good.

            Several members on THT solved the same problem by simply removing the AS valve in their line(s).

            I'm thinking my low pressure pumps cannot over-come the restriction of the ball valve, seat, and the spring tension (spring not shown) within the AS valves. The low pressure pumps had no problem keeping up before. This new system is indentical to the old system except it has shorter fuel line runs and slightly less lift.

            I want to change one thing at a time until I figure it out. Right now, all I want to do is run the fuel level down to and below the 1/2 level. We'll see. I'm carrying a 6 gal portable tank with me for the time being. That'll get me home back here in the bay.

            Thanks for you feed back and suggestions. I'll report back with updates.
            Last edited by flounder; 07-22-2020, 05:56 AM.
            Ole Joe
            Just floundering around
            White Hall & Ocean City, Maryland
            1978 Mako 25

            Comment


            • #7
              I agree with your analysis in theory . I didn't see the info previously that pumping the primers allowed the motors to run. Restriction could be the issue but it is more likely low pressure pump issue OR an air leak between the primer and the motor. If you can easily get to the anti-siphon valves, removing them is straightforward. I have seen recommendations in some service manuals for Optis that recommend that ASVs be removed.
              Here's the part I don't understand; On plane, the gas tends to pool aft making the level of the fuel near the pickup higher than at rest. In that case, the fuel level which might be half at rest can be a lot higher. That scenario would kind of put a kabash on the ASV theory.
              So I am not confident I understand what is going on. Keep us posted and good luck.

              Comment


              • #8
                I did a writeup of the same issue I had with the check valve on my build.

                You may be having issues between the restriction and the amount of lift needed to get the fuel up the siphon tube. When you pull the valve, you should remove the siphon tube and verify it is long enough.

                You've had a month. Did you burn the fuel and verify you fixed it?

                Comment


                • #9
                  No, I've been running off the top half of the tank, topping off when it gets down near 1/2 full.

                  After the last post I decided to finish the season as is. When I take her home, I'm pump off everything above the 1/2 mark and see if the engines run out of gas with the AS valves removed. If they do, I'll focus on the pick up tubes by trying to pump off past the 1/2 mark. If I can pick up the fuel below there, I'II pump all of it out to prove the pick ups are ok. If the pick up tubes prove ok, then I'll focus on.the primary pumps. Again, one item at a time.

                  I'll post results when it gets sorted out.

                  BTW, thanks for checking back.
                  Ole Joe
                  Just floundering around
                  White Hall & Ocean City, Maryland
                  1978 Mako 25

                  Comment


                  • #10
                    I know you're trying to isolate one issue at a time. I strongly encourage you to look at the siphon tube length when you take the valves out. With the A-S valve removed, you should simply have to unscrew the mounting block from the plate and the fitting/tube will come out as an assembly. There's no sense in disassembling it twice if you don't have to. Let me know if you want me to send you a few pictures of mine when you're ready.

                    Comment


                    • #11
                      That would have been the first thing I would have done after removing the AS valves, but I cannot break them free of the hollow fittings that they thread into/through. The pipe dope that they use during assembly must have an adhesive quality in addition to sealing. I called Moeller and spoke with their chief engineer about the problem. He advised me not to work on them too much because “you could end up with a dead tank”. He also said that “it would not be the first time” when I asked if the tank could have been assembled with the wrong pick up tubes…not the responses that give me much confidence concerning their product or customer service. So, I feel like I’m on my own here and removing the pickups may involve a retrofit which Moeller specifically prohibits.

                      BTW, Moeller has published instructions on their removal, so I’m not the first guy to run into this. Following their instruction will not work on my fittings, I guess because of the adhesive quality of the dope. At the torque values they suggest, the hollow fitting still spins within its’ jam nut. Tightening further compresses the polyethylene and I am concerned it could weaken or cut the poly.

                      Because of that, I don’t want to move ahead with their removal until I get home and put the boat in the barn. There I have my shop, tools, and the time to tear into a retrofit…but only if I fail getting the tubes out. I, like you, think the problem is the tubes but want to eliminate all possibilities first.




                      Ole Joe
                      Just floundering around
                      White Hall & Ocean City, Maryland
                      1978 Mako 25

                      Comment


                      • #12
                        mugwump4,
                        And yes, your help with pics and suggestions is appreciated.
                        Thanks
                        Ole Joe
                        Just floundering around
                        White Hall & Ocean City, Maryland
                        1978 Mako 25

                        Comment


                        • #13
                          I'm not sure my pictures will be any better than yours!

                          My only other suggestion is to get a heat gun to loosen up the PTFE sealant (NOT a torch!) and use a open end wrench on the block of the siphon tube and a box wrench on the AS fitting. You won't be able to remove the siphon tube without the AS valve out of the way. Slow and steady pressure, with heat application.

                          I'd use a torque adapter on the tank fitting (see picture), and a 8 point socket on the siphon tube fitting. That way, you can use two breaker bars: one to hold the tank fitting steady, and you can put slow but steady pressure on the siphon tube fitting. Again, heat it up, and have a friend hold the tank fitting while you break it loose.

                          I strongly suspect you've already determined all this; if I can wheeldle a work trip up to Baltimore I'll let you know!
                          Attached Files
                          Last edited by mugwump4; 08-31-2020, 11:09 AM.

                          Comment


                          • #14
                            I can get plenty of pressure on both the jam nut and the 90 degree fitting. Fact is I can easily exceed the 70 ft lbs specified by Moeller. I'm uncomfortable about putting more pressure on them though. The fittings are aluminum and they are tender.

                            I found out just how tender they are when I was removing the AS valves. One of them sheared off inside of the 90. The darn AS valves are bored out to accept the seat assembly making them more tender then a normal barbed fitting that has full wall thicknes.
                            BTW, both the AS valves have been replaced with straight through barbed fittings.


                            I have not tried my heat gun on them since they are mounted in polyethylene/plastic. Not sure it would be a good idea, but may try it as a last resort.

                            If the pickups must be replaced, I expect I'll fabricate a split inner ring and a solid outside ring assembly similar to the assembly you used when you relocated your pick up tube.

                            Once I get home I'll sort it out.
                            Last edited by flounder; 08-31-2020, 09:39 PM.
                            Ole Joe
                            Just floundering around
                            White Hall & Ocean City, Maryland
                            1978 Mako 25

                            Comment


                            • #15
                              Yesterday I realized that it would be foolish to tow the boat all the way home with more than a half tank of gas in it…only to pump it out once there. So, I made a couple long runs in the back bay and removed the gas down to just below the ½ full mark. I am glad to report that, with the Anti Siphon valves removed, the motors perform exactly the way they should. There is no starving at either high RPM or low RPMs.

                              I also have confirmed that the pickup tubes are the proper length. There is a fuel return port fitting in this tank in addition to the two pick up tube fittings. I removed the return port cap and, using a length of coat hanger, was able to chase both pick up tubes to the bottom of the tank by feel.


                              So, unless I experience some starving as the fuel level drops further, I believe the problem is behind me.

                              Thanks to all who offered advice and suggestions.
                              Ole Joe
                              Just floundering around
                              White Hall & Ocean City, Maryland
                              1978 Mako 25

                              Comment

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