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Thread: Fuel Venting from Vent tube

  1. #1
    Senior Member jiott's Avatar
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    Default Fuel Venting from Vent tube

    Today I was flying side by side with a friend's Super Cub and he called me on the radio and said "every now and then I see a white vapor streaming back from your right wing." I asked him to move in closer to my SS7 Kitfox and see if he could tell exactly where it was coming from. He did that and said "every so often I see a spurt of fuel shoot forward out of the fuel cap vent tube on the right wing; then it vaporizes and streams back". I immediately checked my fuel sight gauges and saw the right tank showed full and the left tank about 2/3 full. Needless to say I was concerned and immediately turned for home and landed without incident. By the way, the air was somewhat bumpy today, but not what I consider very bad at all. Both tank fuel caps were on tight and proper with fairly new good (the blue BMW or Mercedes) gaskets. Both vent tubes were clear and open.

    I had taken off for that flight with both tanks full, but not overly full. I have noticed since day 1 (175 hrs ago) that the right tank always seems to stay much fuller than the left during flight. Eventually the left tank shows empty, but the plane keeps on flying just fine as the right tank draws down. I had quit being concerned about it until today. The actual amount of fuel venting out of the forward facing tube must be very small because I have never noticed any more than a normal fuel consumption for any flight.

    In thinking about this, I am sure it is impossible for the forward facing vent tubes to pressurize each tank exactly equally with the corkscrewing airstream coming off the propeller along with other small differences on each side. So a somewhat unequal draindown of each tank is to be expected. But actually losing fuel out the vent really bothers me. I suspect that after both tanks are down to the equivalent of about 1/2 each this burping of fuel out the vent probably stops. I also know that during an aggressive slip it is normal to vent some fuel from the high side wing tank.

    My question to you smart people out there is: Have any of you noticed this during straight & level flight maybe on a somewhat bumpy day? Is there anything that can be done about it other than installing fuel shutoff valves in each tank line? I hate to go the valve route because then I will have to actively manage my fuel system.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  2. #2
    Senior Member jrevens's Avatar
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    Default Re: Fuel Venting from Vent tube

    I'm curious, Jim... do you have a fuel return line that tees off of the fuel line between the fuel pump & the carburetors, & goes back to the header tank? Since the right wing tank is the one that the vent line from the header tank is connected to, I wonder if that has anything to do with it.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
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  3. #3
    Super Moderator Av8r3400's Avatar
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    Default Re: Fuel Venting from Vent tube

    How well in trim does your plane fly? Is the ball exactly in the middle?
    Av8r3400
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    Administrator DesertFox4's Avatar
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    Default Re: Fuel Venting from Vent tube

    Not sure if this will stop your overboarding of fuel but we like to close the four small "vents" in the fuel caps with J.B. Weld. See link to my photo album. http://www.teamkitfox.com/Forums/alb...pictureid=3211


    DesertFox4
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  5. #5
    Senior Member PapuaPilot's Avatar
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    Default Re: Fuel Venting from Vent tube

    The first thing to consider is trim, is the ball centered. I would try to get it centered in cruise. If you want to burn a little more from the right tank then put the ball out to the left a little. This is a trick I used all the time with Caravans and Kodiaks so that I didn't have to turn off a fuel selector for slight fuel imbalances. If I went to a half ball or more it would actually transfer fuel through the header tank to the opposite wing tank.

    Like was mentioned if you have a vapor return to your right tank it could be causing the spouting of fuel through the vent as that vapor builds up and returns into the tank. Once the fuel level goes down a bit you shouldn't lose any more fuel.

    Sorry but I don't understand the reason of DesertFox closing the holes that are outside of the cap's gasket. It seems pointless.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

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    Senior Member jtpitkin06's Avatar
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    Default Re: Fuel Venting from Vent tube

    Wow, this is going to be a guess and by-golly response. There's no way of knowing exactly what the problem is so I'll take a wild stab at it.

    Let's start by agreeing that fuel cannot come out the vent unless the tank is overfilled and fuel is reaching the filler cap. So the question is, "what is causing the fuel transfer.?"

    The possibilities are:

    Flying with a wing low
    Fuel return from a fuel pump (plumbing)
    Imbalance of inlet vent pressure between tanks

    Under normal conditions (not low on fuel) the header tank remains full. The vent line is also full of fuel up to the level of the tank it is vented to. If the system has a "both" or simply "on/off" fuel selector valve it is possible for the fuel to transfer from tank to tank. When a tank gets heavier on one side the condition worsens with time until the wing is lifted by the pilot inputs to prevent turning. As fuel is consumed the fuel imbalance gradually corrects itself. When the tanks are full there is little or no imbalance. When the fuel level drops to create an imbalance there is not enough total fuel to fill the opposite tank to the overflow level. So flying with a wing low, though possible, is not likely the cause.

    Fuel return from a fuel pump is a possibility depending on how you have your system plumbed. One solution is to have any return line go to the header tank instead of the right tank.

    The final area to look is an imbalance in head pressure caused by the ram air vent on the top of each cap. The vents must be adjusted so they are both into the slip stream at the same angle. Even a small angle difference can cause a differential pressure which, under the right circumstances, could push the fuel from one tank to the other. There is quite a bit of pressure in the tanks from the vent lines.

    As a start, I would look at the vents and adjust them so they are the same angle. If the vent tube angles are OK then you might try restrict citing one side with a small cap over the inlet with a slightly smaller hole drilled in it to lessen the ram air pressure.

    Let us know what you find. This is one of those bugs that can be hard to locate.

    Stabbing with blinders on...

    John Pitkin
    Greenville , TX

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    Senior Member HighWing's Avatar
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    Default Re: Fuel Venting from Vent tube

    Quote Originally Posted by PapuaPilot View Post
    Sorry but I don't understand the reason of DesertFox closing the holes that are outside of the cap's gasket. It seems pointless.
    The fuel caps used to be shipped from the factory with those vents welded shut. The reason being that the caps as shipped from the manufacturer are designed with the small dimple internal vents as the normal venting when used unmodified. When the Kitfox factory modifies the cap by adding the pitot vent those are unneeded and in fact the pitot tube is rendered useless if the ram pressure they generate is released through those internal vents.
    Lowell Fitt
    Goodyear, AZ


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  8. #8
    Senior Member jrevens's Avatar
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    Default Re: Fuel Venting from Vent tube

    Quote Originally Posted by HighWing View Post
    The fuel caps used to be shipped from the factory with those vents welded shut. The reason being that the caps as shipped from the manufacturer are designed with the small dimple internal vents as the normal venting when used unmodified. When the Kitfox factory modifies the cap by adding the pitot vent those are unneeded and in fact the pitot tube is rendered useless if the ram pressure they generate is released through those internal vents.
    I believe that the caps, as shipped from the manufacturer, have a couple of holes toward the center that those perimeter holes in effect are connected to. It's those 2 holes that are welded shut when the vent tube is also installed. As long as those 2 holes are closed off properly by the welding, and no pin holes are left, sealing the perimeter holes should not be necessary, as PapuaPilot stated.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

  9. #9
    Senior Member HighWing's Avatar
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    Default Re: Fuel Venting from Vent tube

    Good catch, John.
    Lowell Fitt
    Goodyear, AZ


    My You Tube Channel

  10. #10
    Senior Member jrevens's Avatar
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    Default Re: Fuel Venting from Vent tube

    Jim,

    It may be a dead end, but it would be helpful for me if you could let us know exactly how your vapor/fuel return line is piped, if you have one. Do you have a special fitting with an orifice in the line? As I asked before, does the line start between the fuel pump & the carburetors, and does it go directly to the header tank or tee into the vent line leading to the wing tank, or something different?
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
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