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What can this be? Last week when I flew, my engine (912UL, SN 4005199, 80hp) performed fine. A week later (today) in the hangar there was smell of fuel and tiny drips on the floor. Upon opening the cowl, and removing the air filters, inside each carb, fuel is leaking from one of the tiny holes facing the air filter, drenching the air filters (I can put my finger on that hole and feel the fuel coming out). What is odd is that suddenly both carbs chose the exact same moment to leak fuel exactly the same way while the engine is off (I think this point should tell a lot, but I don’t know what). No fuel is coming from the vent tubes; all fuel is going out the tiny holes facing the air filters.

I mention this because it may be the related. About 20 hours ago I did a complete 5-year rubber replacement. The new fuel pump tested at 5psi throughout the rpm range. The 1-3 floats weighed 2.90gr and 2.85gr, 2-4 floats were 2.92gr and 2.89gr. The fuel levels were correct. I ran the engine for 20 minutes and then cleaned out the float bowls. Everything was running smoothly and I flew for 20 hours.

Could it be that a piece of debris is clogging the fuel pressure bleed-off orifice; and when I shut down, the fuel pump just pressurized the lines and they remained so? But even then, why would fuel come out the hole facing the air filter instead of going out the vent tubes or being held back by the valves? Being a Rans S-7 tail dragger, the engine is canted backward considerably toward the air filter while on the ground.

When I last flew, I shut off the fuel valve (which does work to completely stop fuel, as noted when I remove the gascolator filter). I also fully topped off the fuel tanks (Rans S-7 – high wing) and put the plane away. I came out (as I have for the several months since the 5-year rubber) and this time both carbs were leaking fuel.

I’ve read all the forum results related to my issue but see no one mentioning fuel weeping out the carb hole facing the air filter. Do I take the carbs apart? Or do I run the engine, look things over, and go fly as if it fixed itself (?) and chalk it up to an unknown anomaly?

Thanks,
Dennis
  • Re: Fuel leaking through carburetor air filter

    by » 5 years ago


    Here we go again.  I never figured out an answer to the problem I described above one year ago.    After re-checking float levels and weights (which were correct), I flew for one year, about 50 engine hours without the problem recurring.  Now today, the problem has returned exactly the same as I described above.  Is the seasonal nature a clue?  I don’t know what to do next.  My thinking is back to the return bleed-off orifice, might it be clogged keeping pressure on the needle valves (but then why wouldn’t the needle valves hold anyway).  The air filters on both carbs were dripping wet with fuel and with a small puddle on the floor.  Is this a normal amount of fuel available from a full stroke of the fuel pump assuming it stopped exactly at maximum on the cam?

    After drying the filters with paper towels, I ran the engine for 10 minutes.  It ran fine and once again appears to have resolved itself. Do I keep flying while watching for another repeat while the airplane is hangared?

    I have no idea if this is a safe situation.  But I sure would like to definitively resolve it.

    Any troubleshooting help would be appreciated.


  • Re: Fuel leaking through carburetor air filter

    by » 5 years ago


    During the last time I turned the engine off (days before coming to the hangar to discover the dripping fuel), I recall shutting down differently than normal.  I turned off the petcock, … and then immediately turned off the ignition, seconds apart, but in that order.  Normally I turn off the engine, …. and then turn off the petcock.  Could the closed petcock (while the engine was still turning) have prevented fuel from being drained from the pressurized side of the fuel pump?  And could that, in turn, have caused the fuel to seep out and into the air filters over the days the plane sat in the hangar?  Would there be enough fuel remaining to drip out of the carbs?   Does anyone know?  This seems feasible to me.


  • Re: Fuel leaking through carburetor air filter

    by » 3 years ago


    Hi Denis, did you ever find out why this happened?  I have exactly the same issue but from the right side.... Fuel drips from the idle jet air jet orifice.


  • Re: Fuel leaking through carburetor air filter

    by » 3 years ago


    Paul de Kock wrote:

    Hi Denis, did you ever find out why this happened?  I have exactly the same issue but from the right side.... Fuel drips from the idle jet air jet orifice.

    See response on other post. Most likely an "O" ring issue.


    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


  • Re: Fuel leaking through carburetor air filter

    by » 3 years ago


    Paul de Kock wrote:

    Hi Denis, did you ever find out why this happened?  I have exactly the same issue but from the right side.... Fuel drips from the idle jet air jet orifice.

     

    Good question.  I am glad mine has resolved, but still not sure what was the cause.  Because mine happened from both carbs at the same time, I came up with a possible theory as to why, which you can read in my previous post to see if this might apply to you (or if what I suggest is even possible). 

    In my case, both carbs chose to leak at the same time in the same way, which is why I came up with my own theory when nobody gave any suggestions. My theory is easy enough to test, but I have gone back to my old shutdown order and have forgotten I even had this issue.  I also hate to even test it because when my carbs were leaking, it was very frustrating.

    While that was happening I met one other person with an identical problem who also did not know how to fix it.  We were both excited find someone else with the same problem, but this did not lead to resolution.  Misery loves company.  I mentioned I had posted here without any response.  If I see him again, I will ask if his still leaks or if he found a solution, and if so, I will post what I find out.  I'll tell him one more is like us.

     

    To troubleshoot, I would begin with Roger’s suggestion.

     

    Please let us know what happens.

     


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