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Bloody new fuel pump

in 912 / 914 Technical Questions 13 years ago

  • Re: Bloody new fuel pump

    by » 13 years ago

    Jay,

    You're right. It does back pressure its way back to the wing tank. It really doesn't matter though because it would take the same amount of head pressure to deliver the fuel straight to the wing tank. A pressure regulator would help as far as the over pressure situation is concerned but the main reason for the return line is to prevent vapor lock. My own opinion is that the return line should go into the gascolator or very close to it. Reason being that the gascolator is on the suction side of the fuel system and, to some extent, will even help to pull fuel through the return line.

    Bill.
      
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  • Re: Bloody new fuel pump

    by » 13 years ago

    During low throttle operation, I doubt much fuel would backflow into the main tanks. I imagine that a circular flow would occur, from header tank, through the fuel pump and engine lines, through the check valve and return line back to the header tank. Fuel flow from the wing tank would simply slow down until the engine stops, at which time there would be a small amount of backflow. While the engine is running, there's pretty much always a net flow toward the engine.

    At least that is my understanding of hydraulics. I don't imagine that throttling back would return enough fuel to the header tank enough to completely reverse the fuel flow at the header tank. The plane I'm used to just returns fuel to the main tank.
      
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  • Re: Bloody new fuel pump

    by » 13 years ago

    Thanks Bill,
    My Kitfox Mod IV was built in 99 without a fuel return line. It had been sitting since completed testing about ten years. An A&P mechanic then bought it and cleaned it up. He had the Carburetors rebuilt by a repair facility and I bought the plane at 56 hours total time.
    I flew it for 250 hours the first 9 months I had it and a certified A&P did all the maintenance.

    At 250 hours the engine quit during takeoff and it all pointed to fuel system problems. I landed safely and replaced the entire fuel system. (MINUS WING TANKS) Part of the new install eliminated the gascolator but added a new header tank and three new fuel filters.

    The plane has been venting fuel ever since, and I havn't flown it for anything except maintenance testing, and its annual. I replaced two bad fuel pumps for excessive pressure, and finaly reinstalled the original pump I had been flying when I had the engine failure. Funny, I'm still venting fuel from the fuel vents at low idle. This leads me to believe it is my Carbs. I tried balanceing them with the new carb mate, and even at perfect balance, they vent at low rpm. anything below 2000 starts a vibration that overflows the carbs. I QUIT.

    I pulled the carbs and sent them to a repair facility to see if they are actually adustable within limits and are they actually jetted for an 80 horsepower engine. Is the idle jet different on a 80 horse vs a 100 horse? could that be the deal? I'll install the return line. It seems like it is attacking the symptom not the problem but if it stops venting fuel and helps prevent vapor lock, well, I'll fly it. Right now, I won't taxi it.
      
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  • Re: Bloody new fuel pump

    by » 13 years ago

    Adam,

    You are probably right. The amount of fuel flowing through the return line is so slight that I doubt it would change the fuel flow much if any. The amount of fuel is so small compared to what the engine is using, it would never actually make its way back to the main tank. The return line restrictor is only .014in. Thats a tiny opening.

    Jay,

    Take a look at the 912 installation manual under the support tab. Go to section 14 and it gives a pretty good explanation of how the fuel system should be set up.

    Bill.
    Thank you said by: jay white
      
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  • Re: Bloody new fuel pump

    by » 13 years ago

    Jay,

    Venting fuel from the bowl vent really doesn't have anything to do with jet sizes. It is almost always either overpressure, float level, worn or dirty float valve, or vibration. If you carefully remove the float bowls without spilling any fuel out, lift out the floats and measure the distance from the top of the bowl to the fuel. It should be half an inch. This would indicate that floats and float valve are OK. With the floats in place, the horizontal pin in the float should be just touching the fuel. Overpressure or a bad or dirty float valve or bent float arms will cause the fuel level to be too high.(or too low) Too high will cause venting and a rich mixture. If you very gently lift the float arms, they should be parallel with the carb base and parallel with each other. If you are idling below 1700-1800RPM, that could be the problem. If your EGTs are normal, It may just be the idle. Usually overpressure or bad float valves cause rich mixtures which lower EGTs dramatically.

    Whatever the problem is, I think its going to turn out to be a simple fix.

    Bill.
      
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