Results 1 to 15 of 15

Thread: fuel leak through carburettor

  1. #1

    Join Date
    Oct 2009
    Location
    Wanaka,NZ
    Posts
    289
    Post Thanks / Like

    fuel leak through carburettor

    Since new have always had a slight amount of fuel leaking into the carb air box after shutdown.After 24 hours engine starts even in cold weather the engine starts with no priming.A light spray of fuel is evident on the aircraft undersuface after flight..If the engine is shutdown turning off the fuel cock,none of this happens.Several times recently when I turn on the fuel cock there is a constant flow of fuel through the carburetor.This carb is has done 100 hrs from new.Engine from aerosport power.Ideas please.Thanks,Ron.
    Just finished building a back country cub

  2. #2
    www.SkupTech.com mike mcs repair's Avatar
    Join Date
    Aug 2004
    Location
    chugiak AK
    Posts
    8,341
    Post Thanks / Like
    there's an old thread on here somewhere.... about fuel coming down out of intake tubes/system after shutdown

    you might have float level not right?

    the getting stuck on turning fuel on, is probably because you are shutting fuel cock off and running it dry and leaving it like that, and needle buildup sticking it?????

  3. #3

    Join Date
    May 2014
    Posts
    279
    Post Thanks / Like
    X2 on float level/bad needle valve seat
    I once watched a carb guru work his magic on an O320 carb, he made a gizmo that would screw into the bowl drain threads, it had a rigid clear tube that bent up along side the bowl. It was marked to indicate correct fuel level/float height.
    this enabled him to check float level on a carb while still mounted on the aircraft. Also to confirm float level on pressurized fuel system.
    Not that this will lend itself to this situation but got me thinking about it.
    Last edited by Oliver; 09-16-2017 at 09:32 PM.
    Thanks ron thanked for this post
    Likes mike mcs repair, cubpilot2 liked this post

  4. #4
    cubpilot2's Avatar
    Join Date
    May 2003
    Location
    Anchorage Alaska
    Posts
    497
    Post Thanks / Like
    I just went through the exact thing. After lots of pain we finally found that the primer pump would not seal off completely and fuel flows into the cylinders when shut down and while running. This happened very slow and the fuel would evaporate in the box and leave sticky blue goo. The engine would run a little rich. Alway had fuel stains. Installed a new primer pump and all is well. No more leaks and the fuel consumption has improved.

    We spent a lot of time and money with carb swaps and experimentation. New pumps are expensive. We thought about lapping the seat but didn't want to take time. To test this out; cap off both primer lines (to and from the pump).....we did both anyway. Clean the air box good of any residue and go fly. You should see immediate results.

    The pump problem was not with the o-rings but the needle / seat in the bottom.

    http://www.supercub.org/forum/showth...ery+fuel+stain
    Ed
    Thanks stewartb, ron thanked for this post
    Likes mike mcs repair liked this post

  5. #5
    cubdriver2's Avatar
    Join Date
    Apr 2003
    Location
    upstate NY
    Posts
    7,848
    Post Thanks / Like
    Quote Originally Posted by Oliver View Post
    X2 on float level/bad needle valve seat
    I once watched a carb guru work his magic on an O320 carb, he made a gizmo that would screw into the bowl drain threads, it had a rigid clear tube that bent up along side the bowl. It was marked to indicate correct fuel level/float height.
    this enabled him to check float level on a carb while still mounted on the aircraft. Also to confirm float level on pressurized fuel system.
    Not that this will lend itself to this situation but got me thinking about it.
    Funny, carb guru. Buy a barbed fitting that screws into bottom carb drain hole and clear vynal tubing. Measure float/fuel height on level carb half matting surface.

    Glenn
    Last edited by cubdriver2; 09-16-2017 at 10:10 PM.
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"
    Thanks ron, qsmx440 thanked for this post
    Likes cubpilot2 liked this post

  6. #6

    Join Date
    May 2014
    Posts
    279
    Post Thanks / Like
    Good of point on primer leak. However, Op reported "constant fuel flow thru carb" when fuel valve was reopened.
    im assuming more fuel than could be delivered thru primer system.?


    Quote Originally Posted by cubdriver2 View Post
    Funny, carb guru. Buy a barbed fitting that screws into bottom carb drain hole and clear vynal tubing. Measure float/fuel height on level carb half matting surface.

    Glenn

    "carb guru" status to my mind, occurred when I entered the guys shop. WW 2 era fluid filled diagnostic instruments lining the walls to the ceiling, flow bench, pressure/vacuum chambers and the like.
    He attached his little float level tester to demonstrate the effect that fuel pressure has on float level. What do you suppose happend as he dialed up fuel pressure on MA-4?
    Btw. Float level tester needs to be rigid tube if it's going to be calibrated, but yes, simple to make.
    Last edited by Oliver; 09-17-2017 at 01:04 PM.

  7. #7

    Join Date
    Oct 2009
    Location
    Wanaka,NZ
    Posts
    289
    Post Thanks / Like
    Correct Oliver the flow through the carb would be max demand rate,hence one would probably attribute this to the float stuck in the full down position or sticky float valve?
    Quote Originally Posted by Oliver View Post
    Good of point on primer leak. However, Op reported "constant fuel flow thru carb" when fuel valve was reopened.
    im assuming more fuel than could be delivered thru primer system.?





    "carb guru" status to my mind, occurred when I entered the guys shop. WW 2 era fluid filled diagnostic instruments lining the walls to the ceiling, flow bench, pressure/vacuum chambers and the like.
    He attached his little float level tester to demonstrate the effect that fuel pressure has on float level. What do you suppose happend as he dialed up fuel pressure on MA-4?
    Btw. Float level tester needs to be rigid tube if it's going to be calibrated, but yes, simple to make.
    Just finished building a back country cub

  8. #8

    Join Date
    May 2014
    Posts
    279
    Post Thanks / Like
    Ron,
    sorry to get off track with your post,
    has carb been leaking fuel since installed 100 hrs ago?
    sounds like inlet needle valve not seating.
    carb been refitted with epoxy floats? Leaky floats can give symptoms you describe as well.
    Thanks ron thanked for this post

  9. #9

    Join Date
    Oct 2009
    Location
    Wanaka,NZ
    Posts
    289
    Post Thanks / Like
    Hi Oliver,The high flow through the carb only since I started turning off the fuel cock on shutdown.The reason I was doing this was that I had a slow seep of fuel settling in the carb air box whilst parked for 24 hours.As soon as I opened the cock away it went.
    Just finished building a back country cub

  10. #10
    skywagon8a's Avatar
    Join Date
    Mar 2008
    Location
    SE Mass
    Posts
    7,080
    Post Thanks / Like
    Quote Originally Posted by ron View Post
    Hi Oliver,The high flow through the carb only since I started turning off the fuel cock on shutdown.The reason I was doing this was that I had a slow seep of fuel settling in the carb air box whilst parked for 24 hours.As soon as I opened the cock away it went.
    This sounds suspiciously as though the float is sticking to the sides of the bowl. If the floats are twisted on the connecting plate they will not be aligned with the bowl sides even though the float level may be correct.
    N1PA

  11. #11

    Join Date
    Oct 2009
    Location
    Wanaka,NZ
    Posts
    289
    Post Thanks / Like
    Thanks Pete,We will be dismantling carb next week,interesting this is the second new carb we have fitted in 200 hours..Aerosport power replaced the first one as we couldn't solve the mixture problem with the 0-375 engine.
    Quote Originally Posted by skywagon8a View Post
    This sounds suspiciously as though the float is sticking to the sides of the bowl. If the floats are twisted on the connecting plate they will not be aligned with the bowl sides even though the float level may be correct.
    Just finished building a back country cub

  12. #12
    Gordon Misch's Avatar
    Join Date
    Dec 2003
    Location
    Toledo, Wa (KTDO)
    Posts
    2,579
    Post Thanks / Like
    Suggest you contact (pm) Aviationinfo on this site - he had a difficult-to-diagnose leaking carb issue that is since resolved, but I don't recall what the solution was.
    Gordon

    N4328M KTDO
    My SPOT: tinyurl.com/N4328M (case sensitive)
    Thanks ron thanked for this post

  13. #13
    cubpilot2's Avatar
    Join Date
    May 2003
    Location
    Anchorage Alaska
    Posts
    497
    Post Thanks / Like
    Quote Originally Posted by ron View Post
    Thanks Pete,We will be dismantling carb next week,interesting this is the second new carb we have fitted in 200 hours..Aerosport power replaced the first one as we couldn't solve the mixture problem with the 0-375 engine.
    Sounds so much like what we went through. Changed a needle and seat, Tried a different carb. Had the first carb overhauled. Then my friend bought a factory overhauled carb. This was over two - three years. All had the same issue. Slow learners I guess. Always had the stain and drippings. Never had it "run out" thou. In the winter you would see stain on the snow unless he turned off the fuel

    I'd still test the primer before spending lots of $. It won't cost a dime to find out. If it still leaks then you for sure have a carb issue.

    Good luck
    Ed
    Thanks ron thanked for this post
    Likes mike mcs repair, skywagon8a liked this post

  14. #14

    Join Date
    Oct 2009
    Location
    Wanaka,NZ
    Posts
    289
    Post Thanks / Like
    Solution��Cheap as and as so many of you knowledgeable guys suggested.Took the primer out,cleaned ,crc(plenty of!)fuel lube on the shaft .A light lube on the O rings and refitted.Now no sign of a seepage overnight. The other problem has not reoccurred,that is when I shut the engine down by turning off the fuel cock.When I then turned the fuel cock on again,fuel poured out the carb air box.We suspect the float (new style) was sticking in the demand position,but no proof.Reset float levels and no more fuel spotted belly on the cub.Thanks guys! Ron.
    Just finished building a back country cub

  15. #15
    cubpilot2's Avatar
    Join Date
    May 2003
    Location
    Anchorage Alaska
    Posts
    497
    Post Thanks / Like
    Glad to hear it Ron!
    Thanks for the "feed back"!
    Ed
    Likes Bill Rusk liked this post

Similar Threads

  1. Slow Fuel Leak
    By Fuel in forum Super Cub Sick Bay
    Replies: 9
    Last Post: 08-25-2012, 11:51 AM
  2. Oil Leak & Plexi-fuel stain
    By rrb in forum Super Cub Sick Bay
    Replies: 9
    Last Post: 08-11-2009, 08:47 AM
  3. Fuel leak in Cockpit, J3
    By BlakeH in forum Cafe Supercub
    Replies: 13
    Last Post: 07-27-2007, 04:18 PM

Bookmarks

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •