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Thread: L21B Fuel Gauge Rebuild

  1. #1
    cubnut93's Avatar
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    L21B Fuel Gauge Rebuild

    I am the process of rebuilding the fuel gauge on a L21B that I restored with a friend in 2015. We replaced the fuel selector valve the other day with a Dakota Cub STC'd (this Cub is a certified aircraft) unit and we noticed that the right fuel tank still showed some fuel after we drained both tanks in order to replace the valve.

    I removed the fuel gauge thinking that it had a blockage at the bottom of the assembly. However, when I removed the gauge a small amount of fuel dripped out and the ball fell to the bottom (so I am still chasing that problem)--sorry I digress.

    My question to you is can I epoxy the top fitting to the bushing that holds the glass tube in place??? I am having a problem getting that top fitting aligned so it points in the correct direction. It seems too loose or too tight. I already broke one glass tube while trying to get it lined up correctly (too tight). And I am afraid to leave it too loose because I KNOW it will leak. I have tried using different thicknesses of gaskets but I still didn't have any luck.

    Also while I am on this subject---does anyone have a good process for leak checking the gauge before it is installed in the airplane. I have tried just filling the gauge up with av gas but I feel like I should put at least a few pounds of pressure on the gauge as well. How much pressure should I use??????

    Any suggestions will be greatly appreciated.

  2. #2
    cubdriver2's Avatar
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    All cub gauges will show a little fuel when the tank is empty. The hose on the bottom of the gauge dips down lower then the tank and gauge to clear the strut that goes from the front spar to the rear spar and holds fuel. Never trust the last half inch of fuel in the gauge while flying

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"
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    Eddie Foy's Avatar
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    Quote Originally Posted by cubdriver2 View Post
    All cub gauges will show a little fuel when the tank is empty. The hose on the bottom of the gauge dips down lower then the tank and gauge to clear the strut that goes from the front spar to the rear spar and holds fuel. Never trust the last half inch of fuel in the gauge while flying

    Glenn
    The pea on my PA-18 gauges dissapears with 4 gals remaining.
    Eddie Foy
    "Put out my hand and touched the face of God"

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    skywagon8a's Avatar
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    Quote Originally Posted by cubnut93 View Post
    Also while I am on this subject---does anyone have a good process for leak checking the gauge before it is installed in the airplane. I have tried just filling the gauge up with av gas but I feel like I should put at least a few pounds of pressure on the gauge as well. How much pressure should I use??????

    Any suggestions will be greatly appreciated.
    The FAA requirement for testing fuel tanks is 3 psi.
    N1PA

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    CamTom12's Avatar
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    Quote Originally Posted by cubdriver2 View Post
    All cub gauges will show a little fuel when the tank is empty. The hose on the bottom of the gauge dips down lower then the tank and gauge to clear the strut that goes from the front spar to the rear spar and holds fuel. Never trust the last half inch of fuel in the gauge while flying

    Glenn
    Fact.

    Cross country with the (sleeping) wife and about 3/4 to 1" showing in one tank (other was 3/4 full) and decided to burn it out to see what it indicates when empty. Figured I had a good 30-45 minutes so I didn't wake the wife yet, but I had planned to when it got a little closer to empty.

    She woke up pretty quick on her own about 10-15 minutes later while I was switching tanks. Engine restarted without a hiccup but she was real mad at me for not giving her a heads up. Oops.
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  6. #6
    cubnut93's Avatar
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    Quote Originally Posted by skywagon8a View Post
    The FAA requirement for testing fuel tanks is 3 psi.
    Thanks for that info!

  7. #7
    cubnut93's Avatar
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    So what did the gauge read when the engine quit????



    Quote Originally Posted by CamTom12 View Post
    Fact.

    Cross country with the (sleeping) wife and about 3/4 to 1" showing in one tank (other was 3/4 full) and decided to burn it out to see what it indicates when empty. Figured I had a good 30-45 minutes so I didn't wake the wife yet, but I had planned to when it got a little closer to empty.

    She woke up pretty quick on her own about 10-15 minutes later while I was switching tanks. Engine restarted without a hiccup but she was real mad at me for not giving her a heads up. Oops.

  8. #8
    CamTom12's Avatar
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    Quote Originally Posted by cubnut93 View Post
    So what did the gauge read when the engine quit????
    About 3/4 to 1/2" or so

  9. #9
    skywagon8a's Avatar
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    Quote Originally Posted by cubdriver2 View Post
    All cub gauges will show a little fuel when the tank is empty. The hose on the bottom of the gauge dips down lower then the tank and gauge to clear the strut that goes from the front spar to the rear spar and holds fuel. Never trust the last half inch of fuel in the gauge while flying

    Glenn
    Quote Originally Posted by cubnut93 View Post
    So what did the gauge read when the engine quit????
    Quote Originally Posted by CamTom12 View Post
    About 3/4 to 1/2" or so
    All of you using the original Piper fuel sight level tubes take note of these three remarks. That tube passing under the compression tube between the spars creates a small sump which always has a liquid in it. It could be gas or water. Water freezes in winter. Both when liquid can slosh moving the level indication even when the tank is empty or almost, giving false security. Being able to actually see the fuel level is good. Understanding this particular "indicator" is important. It is also possible that sometime in the planes life that this "sump" has been altered by changing the amount of tubing under the compression tube effecting the amount of liquid in the sight glass. If the location of the sight glass has been relocated it could give a false level indication when compared to another like airplane. Be safe guys.
    N1PA

  10. #10
    CamTom12's Avatar
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    Probably off topic, but I believe you should know what your "real live" empty indication looks like. Only one way to do that, one tank at a time.

    That plus smart fuel management can save your butt if a series of unfortunate events require deviation from flight plan A.
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    Eddie Foy's Avatar
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    I know exactly what my sight gauges are telling me. I have checked several times the run time after the pea dissapears at the bottom. At cruise, I have 30 mins run time at 6GPH or 3-4 gals useable.
    Eddie Foy
    "Put out my hand and touched the face of God"
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    Farmboy's Avatar
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    Quote Originally Posted by Eddie Foy View Post
    I know exactly what my sight gauges are telling me. I have checked several times the run time after the pea dissapears at the bottom. At cruise, I have 30 mins run time at 6GPH or 3-4 gals useable.
    Eddie, inquiring minds want to know.


    Sent from my imitation glass cockpit using SuperCub.Org mobile app

  13. #13
    Eddie Foy's Avatar
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    Quote Originally Posted by Farmboy View Post
    Eddie, inquiring minds want to know.


    Sent from my imitation glass cockpit using SuperCub.Org mobile app
    Guilty of having an inquiring mind.
    Eddie Foy
    "Put out my hand and touched the face of God"

  14. #14
    Farmboy's Avatar
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    Quote Originally Posted by Eddie Foy View Post
    Guilty of having an inquiring mind.
    So how did you check your fuel level several times to be 30 min... ?


    Transmitted from my FlightPhone via Steve's SuperCub.Org mobile app
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    Quote Originally Posted by Eddie Foy View Post
    I know exactly what my sight gauges are telling me. I have checked several times the run time after the pea dissapears at the bottom. At cruise, I have 30 mins run time at 6GPH or 3-4 gals useable.
    Ya but what I want to know is much O2 you're carrying to cruise high enough to get 6 GPH burn with a 320...
    "Don't feed the hipsters"

  16. #16
    Eddie Foy's Avatar
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    Quote Originally Posted by OLDCROWE View Post
    Ya but what I want to know is much O2 you're carrying to cruise high enough to get 6 GPH burn with a 320...
    Depends on how fast you cruise.

    I fly around the flagpole at 5.3 GPH @2200 RPM.
    Eddie Foy
    "Put out my hand and touched the face of God"

  17. #17
    Eddie Foy's Avatar
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    Quote Originally Posted by Farmboy View Post
    So how did you check your fuel level several times to be 30 min... ?


    Transmitted from my FlightPhone via Steve's SuperCub.Org mobile app
    With a time measuring device. The important number is 3-4 gals remaining.
    Eddie Foy
    "Put out my hand and touched the face of God"

  18. #18

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    Quote Originally Posted by Eddie Foy View Post
    Depends on how fast you cruise.

    I fly around the flagpole at 5.3 GPH @2200 RPM.
    That's not cruising that's cruz'n
    "Don't feed the hipsters"

  19. #19
    Eddie Foy's Avatar
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    Low n slow with the door down.

    In the thong that Eaton gave me.
    Eddie Foy
    "Put out my hand and touched the face of God"

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