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Thread: Carb flooding and venting, Kitfox4 912 UL

  1. #21

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    Default Re: Carb flooding and venting, Kitfox4 912 UL

    Dusty,

    Thanks for your response.

    Bob,

    Maybe this is a conspiracy to sale Mr. Gasket products My long story...

    I checked/weighed the floats. They were good. I replaced the float needles even though the original needles and seats looked great and adjusted the float height correctly. I still had the same problem of fuel going overboard.

    I put a Mr. Gasket 0-15 psi analog gauge in the carb supply line to verify the digital gauge, followed by a Mr. Gasket regulator to allow me to control the pressure.

    Without the engine running. the analog gauge and digital gauge registered the same using a primer to supply pressure and I could pressurize to 7 psi without fuel leaving the carb.

    When I ran the engine, the analog gauge was pulsating from 0 to at least 10 psi rapidly (needle was a blur). Starting at 6 psi on the regulator, I went down in half psi increments to 3.5 psi and the carbs quit flooding.

    I think the high pressure pulses are unseating the float valves and maybe another fuel pump is due or maybe a fuel pressure dampener... - but I don't think I should have to run a regulator.

    I would appreciate anyone's thoughts about replacing the fuel pump, pressure dampener, etc.


    Charles

  2. #22
    Super Moderator Av8r3400's Avatar
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    Default Re: Carb flooding and venting, Kitfox4 912 UL

    This has been an interesting discussion.

    I have the "new" style mechanical fuel pump, I purchased it back in 2014-2015 time frame. My Dynon D180 has a fuel pressure sensor and readout on the display. I constantly show 3-5 psi of fuel pressure.

    I would agree that 7+ psi is excessive and could very well overpower the floats.
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
    YouTube Videos

  3. #23
    Senior Member Dave S's Avatar
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    Default Re: Carb flooding and venting, Kitfox4 912 UL

    Bob & Charles,

    I think, with the information known now, there is very little doubt about the excessive pressure being the cause of the fuel overboarding. FWIW, Rotax has snatched defeat from the jaws of victory a couple times in the past by shipping fuel pumps which produced excessive pressure. If I recall, there was a rotax document back a long time ago explaining the need to replace certain pumps because of excessive pressure. They have screwed up factory supplied fuel hoses between the pump and carbs too - but that is another issue.

    I don't buy the angle that the aircraft sits at on the ground argument. There is a rotax spec (don't recall where at the moment but it may be in the engine installation or operator's manual) which indicates an angle deviation from vertical far in excess of what was measured - don't recall the number but I believe it was somewhere in the neighborhood of 40 degrees - the carbs are pretty darn good. Considering that Kitfoxes can climb at deck angles in excess of taxi deck angles without problems like this - oh well. After all, its an airplane engine that normally spends time in less than vertical positions.

    Another little known tidbit is that unless a person knows for sure how a gauge or electronic readout of fuel pressure is arrived at, there is some possibility that the data is displayed in dampened format (either electronic averaging or having a restrictor orifice to reduce the vibrating needle syndrome) Your gauge which reads the high and low peaks apparently does not have any damping function and may be reading true to what is happening.

    A correct, direct reading diagnostic pressure gauge does take the guess work out of things like this.
    Not a bad habit of using such a gauge for annual condition inspections as well as whenever a pump is changed.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  4. #24

    Join Date
    Oct 2020
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    Camp Hill, PA
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    Default Re: Carb flooding and venting, Kitfox4 912 UL

    Hi,
    We think we solved the problem. We checked the pressure the old Mr Gasket was providing, 4psi. Then we connected the new Mr Gasket, set at # 3
    position which gave us 4psi. The carbs didn't flood, but there was some dripping from the vents at idle. The engine ran smoothly, except at idle. which was slight. Regardless of what others have said, we think that the position of the taildragger is the cause of our difficulty. And the regulator seems to be the answer. We have to put it back together tomorrow. Plan to fly it on Saturday. I'll keep you up to speed.
    Bob

  5. #25

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    Oct 2020
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    Camp Hill, PA
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    Default Re: Carb flooding and venting, Kitfox4 912 UL

    I flew this morning. There's some tweeking to do, with regard to idle speed. The engine ran well.

  6. #26

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    Apr 2012
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    Lexington,NC
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    Default Re: Carb flooding and venting, Kitfox4 912 UL

    Bob,

    Just following up with my end results.

    I installed the new pump (893115, same as the old pump), with an analog gauge, fuel regulator and another analog gauge in the pump output line. Carbs are balanced.

    The new pump still pulsated as expected but the pulsations were not as extreme. With the regulator at maximum pressure (7 psi), approx. 5.5 psi was seen at the carbs.

    Like you experienced, I was still getting overflowing at idle.

    I lowered the regulator in half psi increments until the overflowing stopped (3 psi) and I ran the engine through the entire rpm range with the pressure varying from 3 to 3.5 psi.

    The regulator stays.

    Charles

  7. #27
    Birdseyeview's Avatar
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    Default Re: Carb flooding and venting, Kitfox4 912 UL

    I have the same issue going on so I'm also looking for answers too and it looks like you got your answer with a new pressure regulator. However, I'm curious if you came to the conclusion that the average pressure was just too high and the regulator brought that down or do you suppose it was the pulsations were too high and the regulator just calmed that down too? When you measured the pressure and saw the pulsations how did you get a stable overall pressure reading? If the issue is ultimately caused by high pulsations might a fuel pressure damper be a simpler, more compact and more reliable solution than a regulator? I'm not questioning your solution, just curious about details you discovered along the path to your solution.
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  8. #28
    Senior Member
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    Default Re: Carb flooding and venting, Kitfox4 912 UL

    Quote Originally Posted by cwsnyder2001 View Post
    Bob,

    Just following up with my end results.

    I installed the new pump (893115, same as the old pump), with an analog gauge, fuel regulator and another analog gauge in the pump output line. Carbs are balanced.

    The new pump still pulsated as expected but the pulsations were not as extreme. With the regulator at maximum pressure (7 psi), approx. 5.5 psi was seen at the carbs.

    Like you experienced, I was still getting overflowing at idle.

    I lowered the regulator in half psi increments until the overflowing stopped (3 psi) and I ran the engine through the entire rpm range with the pressure varying from 3 to 3.5 psi.

    The regulator stays.

    Charles


    I can't remember, did you have a fuel return line or not? And if you do, where does it go to? Thanks, JImChuk

  9. #29

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    Apr 2012
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    Lexington,NC
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    Default Re: Carb flooding and venting, Kitfox4 912 UL

    Jim,

    I have a return line routed back to the return inlet on the header tank.

    Larry,

    In the test with the old pump, I only had the pressure gauge before the pressure regulator and just interpolated the average pressure while watching the wildly swinging needle and digital gauge.

    In the test with the new pump, I added a fuel gauge after the regulator. The pressure after the regulator was "calmed down" (steady, no fluctuations). I attempted higher pressures on the regulator (and would really like to not have it in the system at all) but the carbs (left carb first) would start dribbling at idle - particularly cold idle as the engine is a little twitchier.

    Roger Lee on rotax-owner.com mentioned a dampener is some of his responses to others on this issue but its location was said to be before the fuel pump which did not make sense in my situation. It seems I need to lower the pressure anyway.

    I also checked the location of the fuel pump vent by blocking and unblocking it with no effect.

    Charles

  10. #30
    Senior Member Slyfox's Avatar
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    Default Re: Carb flooding and venting, Kitfox4 912 UL

    Quote Originally Posted by cwsnyder2001 View Post
    Bob,

    Just following up with my end results.

    I installed the new pump (893115, same as the old pump), with an analog gauge, fuel regulator and another analog gauge in the pump output line. Carbs are balanced.

    The new pump still pulsated as expected but the pulsations were not as extreme. With the regulator at maximum pressure (7 psi), approx. 5.5 psi was seen at the carbs.

    Like you experienced, I was still getting overflowing at idle.

    I lowered the regulator in half psi increments until the overflowing stopped (3 psi) and I ran the engine through the entire rpm range with the pressure varying from 3 to 3.5 psi.

    The regulator stays.

    Charles
    just to let you know. the facet pumps for these rotax 912uls is rated at 3-4.5. This a facet pump that Vans aircraft uses in the rv12. so with that I'm sticking with the fact that you want 3-4.5 pressure at the carbs, above that and you will overcome the needles and have flooding. have a nice day
    steve
    slyfox
    model IV 1200-flying
    912uls
    IVO medium in-flight
    RV7A-flying
    IO-360
    constant speed prop

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