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Thread: Fuel Leak

  1. #1

    Fuel Leak

    I have a fuel leak (in a Sundowner) first noticed when I saw blue on the nose gear. I'll describe what I know. Am asking for some guidance.

    ** Cold, cold start required lots of priming and cranking.
    ** Cold battery eventually needed a jump start.
    ** After brief flight, saw blue stains on nose gear.
    ** Stains ran down gear, not towards tail. Belly is clean.

    I took off the cowling the next day and put Kleenex under each fitting of the primer system and fuel lines.

    ** No stains anywhere when engine is primed.
    ** Fuel drips slowly from a "fitting" on the underside of engine
    when electric boost pump is on.

    I don't know the purpose of the fitting. It looks like a pipe fitting that screws into a flat place on the underside (carburetor ?) of the engine. The other end of the fitting is a very small diameter tube (kinda like a 2-3" long hypodermic needle).

    The fitting reminds me of the drain screw on oil pan of my car, except this one has a tube coming out of the center of it. Fuel is dripping out of the end of that tiny tube when the boost pump is on.

    Would someone kindly tell me what this is, and advise why it leaks?
    Do I leave it alone?
    Do I have an A&P torque the fitting?

    Thanks!
    Mark Anderson
    '73 Sundowner


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  2. #2

    Fuel Leak

    >I have a fuel leak (in a Sundowner) first noticed when I saw blue on the
    >nose gear. I'll describe what I know. Am asking for some guidance.
    >
    >** Fuel drips slowly from a "fitting" on the underside of engine
    > when electric boost pump is on.

    Sounds like the "Sniffle Valve" on the sump that is intended to allow
    flooded engines to drain away the excess liquid fuel in the intake
    manifold. It is doing its job...

    The carburetor may have a float problem, in that the boost pump can lift
    the needle off the seat and allow the carb to flood the intake when the
    boost pump is on. It should NOT do this.

    >I don't know the purpose of the fitting. It looks like a pipe fitting
    >that screws into a flat place on the underside (carburetor ?) of the
    >engine. The other end of the fitting is a very small diameter tube (kinda
    >like a 2-3" long hypodermic needle).

    It is supposed to have a small rubber hose to drain the fuel away from the
    inside of the cowl. Many of them are LONG GONE after 25-35 years.

    >Do I have an A&P torque the fitting?

    Won't do any good to tighten the fitting in the intake (looks like the oil
    sump), since it is NOT leaking around the threads, and the "leak" from the
    tube is EXACTLY what it is designed to do.

    When the engine starts, the manifold vacuum sucks the valve closed, and the
    fuel is pulled up into the cylinders.

    When the engine stops, the valve "falls" back into its normally open
    position and any liquid fuel in the bottom of the intake can drain out.

    Get your carb and the boost pump checked out. No chance someone put a Fuel
    Injection boost pump on the plane, is there? The High Pressure pump for
    fuel injection would cause this problem, and there would be nothing wrong
    with the carburetor...

    Bob Steward, A&P IA
    Birmingham, AL



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