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Thread: Fuel flow problem

  1. #1

    Join Date
    Mar 2019
    Location
    Bonita Springs, FL
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    28

    Default Fuel flow problem

    I'm having a problem which seems to defy physics and logic, or maybe it's just me, there must be a simple fix for this. My aircraft just does not want to feed fuel from the left tank, the right tank feeds fine. Nothing is obstructed or kinked, the fuel cap vents are clear, and the finger strainers are perfectly clean.

    Upon some taxi testing the other day trying out the new Shock Monster tailwheel, I started with equal fuel, about 3 gallons in each tank. After a number of high-speed taxi runs, I noticed the fuel visible in the vent line was below the wing, confirming that the right tank was completely empty, and the left tank was still showing the same as when I started. I shook the plane by the wingtip and jiggled the fuel line around quite a bit, still no fuel flowing. Back in the hangar (my garage) I did several rounds of draining the fuel from the lower sump to the point that the header tank was partially empty, then adding fuel to the tanks a gallon at a time to observe the flow into the header tank.

    Each time I put fuel in the left tank, it would not flow into the header tank, unless I disconnected the rubber hose from where it connects to the metal portion of the line. Gas would immediately gush from the hose out of the wing tank, I would quickly reconnect the line, and then flow quite freely into the header tank and begin cross flowing up the vent tube and fuel line on the right tank. Upon draining from the lower sump again, the right tank would drain completely as the header tank went partly dry, but the left tank still had the original amount of fuel. Repeat the process, as soon as I disconnect momentarily the rubber hose from the metal line, fuel would flow. Over and over, fuel would flow from the left tank only momentarily. As soon as fuel stops moving into the header tank, the left tank stops flowing and the right tank works fine.

    As can be seen in this photo, this is the configuration of the fuel lines at the header tank as it was put together by the original builder. Each wing tank hose connects to a metal line which both come together at the blue AN tee connector in the center, and the resulting single line (the rubber hose in a loop) goes to the header tank. It seems like some sort of air lock, but how? It's about as simple as a lawn mower, just 2 fuel lines feeding a central header tank. The fuel should just level itself between the 2 tanks. The looped hose seems like a strange configuration, but it really should not make a difference being gravity fed from above and it works fine.

    I saw in the online build manual for the Model 7 that the header tank has separate fittings for each wing tank hose, and the vent fitting in the middle. Could the tee connector somehow be causing this problem?

    IMG_7071.jpg
    Scot Trueblood
    Bonita Springs, FL

    Kitfox IV in Phase 1 Flight Testing

  2. #2
    Senior Member jiott's Avatar
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    Jun 2010
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    Portland, OR
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    Default Re: Fuel flow problem

    Try eliminating that long looping hose which has a definite air trap up high. Make a short connection from the tee to the header tank entrance, making sure fuel is always flowing downhill. I would also try to move the vent fitting to the very top of the tank.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  3. #3
    Senior Member rv9ralph's Avatar
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    Aug 2014
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    Greenleaf, ID
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    618

    Default Re: Fuel flow problem

    What is the line exiting the top of the tank on the right side and going straight down. I think that that port should be for the right tank to feed the header tank. The left tank should feed the header through the port that the tee feeds.

    I would suggest also, to drain both tanks and check for flow restrictions in the tee fitting.

    Good luck.

    Ralph

  4. #4
    Senior Member
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    KDKB (Dekalb, Illinois)
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    Default Re: Fuel flow problem

    Am I seeing things or is that Header Tank mounted upside down?

    Maybe I just don't know how the IV Header looks ...

    Jeff

  5. #5
    Senior Member jrevens's Avatar
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    Arvada, CO
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    Default Re: Fuel flow problem

    It looks like you might have a recirculating line (with the check valve that can be seen) feeding the port that your right tank is supposed to feed into. Then your two tank feeds are tee’d together and connected to the port designed for your left tank only. I don’t think it’s right to tee those two tank feeds together like that. The slightest difference in pressure, in this case probably the right tank is greater than the left one, could possibly decrease or stop the flow from the left one. That pressure difference could be due to any number of factors such as line length, routing, fuel levels in the tanks, possible obstruction, etc. I could be wrong, but I think you need to separate those two tank feeds and bring them into the separate ports on the tank. The recirculating line can be tee’d to the vent line on the header tank. Then again I might be completely wrong… it’s late & I’m tired.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

  6. #6
    Senior Member Dave S's Avatar
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    Oct 2008
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    St Paul, MN
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    Default Re: Fuel flow problem

    The header tank does not look like anything supplied by Kitfox. All of the header tanks supplied by the factory I have seen, plastic or aluminum, have three fittings on the top of the tank all pointing straight up, one in the center to bleed the header tank of air/vapor and one on each side of the bleed fitting to receive fuel from each wing tank. Honestly, the header tank in the photo looks like some adaptation from somewhere else a previous owner may have replaced the often drooling original plastic header tanks with. The one in the photo captures air/vapor in the top with no way for it to exit. Best fix is the current model aluminum header tank supplied by Kitfox and route the tubing exactly as the manual shows.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  7. #7
    Senior Member
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    Default Re: Fuel flow problem

    What Dave S said! Follow his advice with the new aluminum header tank and NO tee's.

  8. #8
    Senior Member Eric Page's Avatar
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    Jun 2020
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    Toledo, WA
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    Default Re: Fuel flow problem

    Scot, I have a Series 5 plastic header tank that's taking up space in my garage. It has multiple connection bosses on the top surface, as others have described above. It had brass fittings installed by the original builder of my plane but has never had fuel in it. I removed the fittings and cleaned old sealant from the threads.

    The tank is essentially worthless from a resale standpoint, so I would likely just end up throwing it away. If you find that re-routing your fuel lines doesn't help and you'd like to try my tank, I'd be happy to send it to you. Just PM me your address.
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
    Map of Landings

  9. #9

    Join Date
    Dec 2009
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    Lawrenceville, GA
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    180

    Default Re: Fuel flow problem

    I've got that same plastic header tank in my junk pile. It can work but tended to leak evidenced by the fuel stains on it. I think it was used on older model 3/4s.

    My KF3 had a round aluminum header tank behind the instrument panel. It had a pump that could feed it and a shut off valve at each wing tank. Sometimes that header tank wouldn't fill w/o the pump esp. on take off. The vent tube back up to the right tank was clear with a red ball in one segment so you could see whether or not gas was in it. This all worked but you had to think about fuel management. The current design is *much* better.

    As others have said the problem with the pictured arrangement is that there isn't a good way for the header tank to fill and and the air to cycle back up to the main tank. There should be one line from each of those 3 ports. One each to the outlet of the left and right tank. Last one up to the side vent on the right tank.

    Best would be to replace it with the current aluminum tank that mounts low behind the right seat.

    My 2 cents,

    Mark Napier
    Building a KF IV Classic

  10. #10

    Join Date
    Dec 2009
    Location
    Lawrenceville, GA
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    180

    Default Re: Fuel flow problem

    Do you have a fuel return from the engine going through that one way valve? If that's what that is then yeah you have to keep it.

    If you have to put in a tee for that I think it should be in the fuel line that goes up to your left tank. The right side fuel line and esp. that right vent line should be strait runs with no low spots.

    FWIW


    Quote Originally Posted by napierm View Post
    I've got that same plastic header tank in my junk pile. It can work but tended to leak evidenced by the fuel stains on it. I think it was used on older model 3/4s.

    My KF3 had a round aluminum header tank behind the instrument panel. It had a pump that could feed it and a shut off valve at each wing tank. Sometimes that header tank wouldn't fill w/o the pump esp. on take off. The vent tube back up to the right tank was clear with a red ball in one segment so you could see whether or not gas was in it. This all worked but you had to think about fuel management. The current design is *much* better.

    As others have said the problem with the pictured arrangement is that there isn't a good way for the header tank to fill and and the air to cycle back up to the main tank. There should be one line from each of those 3 ports. One each to the outlet of the left and right tank. Last one up to the side vent on the right tank.

    Best would be to replace it with the current aluminum tank that mounts low behind the right seat.

    My 2 cents,

    Mark Napier
    Building a KF IV Classic

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