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912UL Runnability issues

in 912 / 914 Technical Questions 11 years ago
I have been having issues with this engine for quite a long time- long story short bought plane last year with 150 hours on engine, has never run right, rebuilt carbs with new floats, needles, intake manifold rubbers, synced with carbmate. Rebuilt the gearbox completely(gearset,dog gear, bearings) Here is my problem- the engine runs smooth for a while, then gets progressively worse. Will not idle smoothly below 2500, pulled the plugs and front two cylinders upper and lower are black, rear upper and lower look about right(slightly brown). Air fuel is at 1 and 1/2 turns, needle set at 3rd clip from top. I am at 2000 feet elevation. Running a warp drive prop, pulls 5200 on climb out, turbine smooth 3000 up. Any ideas will be appreciated. This is on a Kitfox model 4/1200.

Thanks,

Scott

  • Re: 912UL Runnability issues

    by » 11 years ago

    Hi Scott,

    Just a thought.

    Some of the KitFox's have and or had vibration issues due to the rubber engine mounts. Harmonic vibration may be an issue for the carbs. You may have harmonics at your problem rpm. It would wear down the needles and cause carbs to vent. They would trim the aluminum sleeve in the middle of the rubber mount and tighten them down more than factory. Contact Kitfox and ask exactly what they do.

    p.s.
    I take it the carbs sync okay?

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

      
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  • Re: 912UL Runnability issues

    by » 11 years ago

    Roger- thanks for the reply. The carb sync is right on, I even eliminated the Kitfox linkage and went to a McFarland cable for more accurate sync, I have done SB#54 which is the shortening of the spacers to .93 inch and adding washers per Kitfox. A friend of mine suggested the float level might be too high because it is a taildragger, however when I adjusted them down slightly I only made it to about 300 feet agl before I lost power. I am thinking it has to be somewhere in the idle circuit because both ignitions are the same, and from midrange(2500) on up it pulls cleanly. Has anyone ran a smaller idle jet? I don't see any other options other than the stock 35-
      
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  • Re: 912UL Runnability issues

    by » 11 years ago

    What's the full story on the engine? It has 150 hours on it... but had it spent long periods of time sitting in a hangar before you bought it?

    If it's running turbine smooth above 2500 RPM, but rough below, it's making me think the idle circuit is where you should look next. The idle system has an intake port on the airbox side of the carb where air enters... then the air passes through a passageway inside the carburetor, where it meets the idle jet. The Idle Jet has an extremely tiny hole in it, which is has to suck fuel up from the float bowl during idle. The fuel gets sucked up into the jet due to low manifold pressures on the engine side of the throttle butterfly valve. The idle jet is where this fuel mixes with the air via the passageway I just mentioned. The fuel and air mix in the diffuser portion of the idle jet. From there, the fuel air mixture passes through a passageway to the idle mixture screw, and then out a tiny port hole on the engine side of the carburetor. This whole system relies on low manifold pressure to suck the fuel and air through the idle circuit of the carburetor. Therefore, at higher manifold pressures (i.e. above 2500 RPM) this idle system is deactivated.

    It is a common problem with airplanes that have been sitting for long periods of time with car gas in them (i.e. water suspended in the fuel - causing corrosion of the jets), which can gunk up this idle circuit with either bits of corrosion or goo from the fuel. I can't stress enough how tiny the idle jet hole is. I would recommend pulling pulling the idle jet and making sure it's clean. Also, with compressed air, I would spray into the intake port of the idle system to ensure there's nothing stuck in the passageway.

    I'm attaching a picture of the idle jet, as well as a link to a small youtube video I made a while back explaining the idle system. Look at the picture of the idle jet... this is what the fuel has to travel through... if the fuel can't get through this nearly microscopic hole, you can have a rough idle (2500RPM and below). This might go undetected during a carb sync, because the carb sync only tells you about manifold pressure, not fuel/air mixture.

    http://youtu.be/tqXpRuH5mOg

    Good luck,

    Tom Johnston
    IdleJet.jpg (You do not have access to download this file.)
      
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  • Re: 912UL Runnability issues

    by » 11 years ago

    I agree what Tom says, but remove the carb top and remove the diaphragm before you use compressed air on the carb.

    You can end up dislodging or damaging the diaphragm.

    I would remove the carbs and completely strip and rebuild them.

    Mark
      
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  • Re: 912UL Runnability issues

    by » 11 years ago

    When I have to clean this jet. I use a 16 ga. wire about 3" long. Strip the insulation off about half way down. Then bend out a single strand of wire. Move it back and forth and around in the hole. It's a perfect fit for the idle jet hole and will not cause damage to the jet. Flush it after this with some carb cleaner followed by air. The idle jet holes are tiny and leaving old fuel in for months to years can clog them up.

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

      
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