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Ignition module won't shut off

in 912 / 914 Technical Questions 12 years ago

  • Re: Ignition module won't shut off

    by » 7 years ago

    Hi Joe,
    That theory won't work. When owners pull the throttle back the carbs still get fuel because they are set to idle somewhere between 1600-1900 rpm so they will always get fuel.
    When the ignition is shut down there is no electrical impulse to fire the modules and no energy to the coils and then the plugs. No fire and it's not going to run. Cylinders are not that hot to auto ignite the fuel or you'd have a issue under normal running for all owners.

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

      
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  • Re: Ignition module won't shut off

    by » 7 years ago

    Roger I appreciate your input very much. I am still trying to wrap my brain around why this engine will not shut off in flight. For the last couple of flight it will not shut off on the ground after landing as well. The fuel has to be cut off and allow the fuel to run out of the carburetors before it will stop.

    But the theory that the carburetor floats at least one of them maybe be heavy and allow greater than idle fuel amounts to pass into the combustion chamber still make sense to me. I don't know your experience in flying motor gliders, where ones flies to altitude and shut down the engine and then feature the propeller. Normally one slows down the aircraft to below 60kts, turn off the modules, and the propeller slows to a stop, feather the propeller and soar or glide around awhile. However there is still airspeed that causes the propeller to windmill and assist the continued turning of the pistons. With the windmill effect and any combustion taking place the engine continues to run, very uneven, but running never the less.

    Assuming that the modules or properly grounded, and or in fact shut off and not producing any spark to the plugs. If a float or floats are heavy and allowing the needle valve to bypass more fuel than it would allow at the normal idle setting, then fuel entering the hot chamber, I think would have a tendency to ignite. I have personally experienced the phenomenon myself in the aircraft in question. We are going to do the carburetor overhauls to see, they are do for overhaul anyway. But thanks again Roger, and if you have more please by all means, say it out loud so we can all hear it. Joe
      
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  • Re: Ignition module won't shut off

    by » 7 years ago

    The Floats do not control the amount of fuel entering the carburetor proper.
    They control the amount of fuel entering the float bowls.
    The Jets and Needles control the amount of fuel Exiting the float bowls and enter the air stream.

    The float level in the carb bowls has a minor effect on the richness of the mixture.
    As you know, shutting off the fuel to the carbs does not instantly stop the engine.
    It takes a while for the fuel level in the carb bowls to drop low enough for the engine to stop.
    As the level drops, the engine continues to operate normally for quite some time.

    Heavy Floats will only raise the level of the fuel in the bowls a small amount.
    If the floats were so heavy as to not allow the float valves to close, the carbs would be overflowing onto the ground.

    The carburetor and floats are not the root cause of the issue.
    A rich mixture may be causing carbon buildup, but the rich mixture is not causing the dieseling.

    The engine operates normally with the ignitions switches ON.
    When the switches are turned OFF, the engine diesels-on running roughly.
    This confirms that the plugs stopped firing with the switches OFF.
    If the switches were actually ON, the engine would continue running like they were ON.
    You would not see any change when operating the switches.

    If you have some hot carbon spots in one or more cylinders, dieseling occurs.
    The hot carbon is there during normal operation.
    It does not cause any problems because the spark plugs normally fire well before Top Dead Center (TDC) where the compression is highest and the likelihood of dieseling will occur.
    Buy the time the hot carbon would ignite the fuel, it is already burning. No problem.

    When the ignition is shut off, the carbon clinkers get their chance.
    The piston continues to compress the fuel load up to TDC where the clinker is able to ignite it.
    It is Too little, Too late. You get a weak ignition that may still be burning when the exhaust valve opens and produces an extra loud bang in the exhaust.
    This is not happening in all four cylinders. It may only be one, which is why the engine runs so roughly and weakly.
    Overall the engine runs rough, loud and weak.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

      
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  • Re: Ignition module won't shut off

    by » 7 years ago

    What is the cause when the Ignition module stops working?
      
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  • Re: Ignition module won't shut off

    by » 7 years ago

    Ignition modules may have different causes for a failure. Some are fixable and some aren't.
    If a wire going to the module breaks or if a ground wire is lose or broken then they can have a temporary failure that you can fix. Commonly caused by vibration or bad ignition switches.
    The modules are just electrical components made by man so they can go bad on some engines and then they need to be replaced. Nothing in the electrical world is ever 100% perfect 100% of the time. I know that's miserable when it cost $1K a pop.
    Another module killer is physical heat. 170F is a killer. Mine in cooler air flow never got over 149F. One way to keep an eye on things is to put temp strips on the modules and the voltage regulator.
    Occasionally an trigger coil will come loose of get bumped and put the air gap out of spec. That can be adjusted.

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

      
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