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Thread: Engine missing on "L" ignition switch setting

  1. #1
    Birdseyeview's Avatar
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    Default Engine missing on "L" ignition switch setting

    The ignition modules are mounted on the top of my 912 ULS engine as described in my Kitfox 6 build manual. Although this is a 2001 vintage engine, at present I only have 59 hours of operation on it. On a previous recent flight the engine started fine and ran smoothly with no missing during run up mag check or during flight. A few days later on 08Oct22 the engine started and warmed up normally but during a run up mag check at 4000 rpm the engine began missing randomly. It seemed minor at first so I took off and while in the pattern it happened again at around 4500 rpm (with the mag switch set on “BOTH”). After landing another mag check was performed and the random missing seemed to be the worst when the ignition switch was set on “L”. During the mag check at 4000 rpm the engine rpm dropped around 150 rpm (from the “BOTH” setting) for both the “R” and “L” switch setting, which seemed normal. I checked the continuity of the kill wires from the ignition modules “A” and “B” and they were both functional. I also confirmed that the Ignition module “A” is connected to the ignition switch “L” terminal. I assume that when the ignition switch is set on “L” the “A” module is turned on and the “B” module is turned off via the kill wire grounding that module. The ignition switch was removed and checked for proper continuity at its various settings and it checked out as functional.

    IMG_9127.jpgIMG_9128.jpg



    I can’t be sure yet but I suspect that the current missing is due to an ignition issue of some type since it came on so suddenly and it seems more isolated to the “L” side setting on the ignition switch. Any suggestions?


    In order to rule out any potential fuel supply issues a subsequent check of the fuel level in each carb fuel bowl checked out as normal and a look at all the spark plugs showed the normal tan color as well. Both carbs have recently been overhauled and have since been performing very well. After reinstalling the plugs I reran the engine in order to check the fuel pressure and it was within the normal range. I even changed the fuel pump from the original Pierbug pump to the latest Rotax pump p/n (thinking that the old pump may be defective somehow). I also checked the gas tanks and header tank for water with none found. None of these checks showed any potential cause of the engine missing due to fuel supply anomalies.


    Note: A previous carb issue a number of flight hours earlier was experienced that caused an engine missing (stumble) due to air leaks at the carb to manifold rubber interface (resulting in extreme lean operation). This was remedied and the sound of that engine stumble was different (more severe) than the missing described above.


    A follow on operational check for spark in cylinder #2 with a strobe light displayed a flashing strobe for both the top and bottom plugs. None of the other plugs were checked for operational spark at this time. Should I do this test on all the other plugs while running the engine?

    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  2. #2
    Senior Member 109JB's Avatar
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    Default Re: Engine missing on "L" ignition switch setting

    Have you changed the plugs?
    John Brannen
    Morris, IL
    Sonerai IIL (Single Seat)
    Kitfox 3/4 1050 - Rotax 582 (Back Flying and sold)
    Kitfox IV 1050 - Rotax 582 (sold)
    Kitfox IV 1200 Speedster - Rotax 912 UL (project)
    Piper Twin Comanche (Sold)
    Glasair 1 FT (Waiting to start)

  3. #3
    Birdseyeview's Avatar
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    Default Re: Engine missing on "L" ignition switch setting

    Good idea - I have not changed the spark plugs. I was expecting to get at least 100 hours from my current set but they are easy and cheap enough to change now at 59 hours. Just curious, are my symptoms typical of a bad plug?
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  4. #4
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    Default Re: Engine missing on "L" ignition switch setting

    Check your spark plug caps too. I think Rotax says 9 pounds pressure to pull off. I had a couple that were pretty loose on a low time engine. A mag check revealed one had come off. It may be something that simple. Good luck.

  5. #5
    Senior Member 109JB's Avatar
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    Default Re: Engine missing on "L" ignition switch setting

    Quote Originally Posted by Birdseyeview View Post
    Good idea - I have not changed the spark plugs. I was expecting to get at least 100 hours from my current set but they are easy and cheap enough to change now at 59 hours. Just curious, are my symptoms typical of a bad plug?
    Yes, a bad plug can manifest as a miss. A friend just had a bad Champion aircraft plug exhibit the same symptoms. Hurts a bit more when talking about a $200ish fine wire plug.
    John Brannen
    Morris, IL
    Sonerai IIL (Single Seat)
    Kitfox 3/4 1050 - Rotax 582 (Back Flying and sold)
    Kitfox IV 1050 - Rotax 582 (sold)
    Kitfox IV 1200 Speedster - Rotax 912 UL (project)
    Piper Twin Comanche (Sold)
    Glasair 1 FT (Waiting to start)

  6. #6
    Senior Member Slyfox's Avatar
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    Default Re: Engine missing on "L" ignition switch setting

    [/FONT][/COLOR]
    A follow on operational check for spark in cylinder #2 with a strobe light displayed a flashing strobe for both the top and bottom plugs. None of the other plugs were checked for operational spark at this time. Should I do this test on all the other plugs while running the engine?

    [/QUOTE]

    absolutely do a test on each cylinder with a timing light hooked up. when it misses you should see it on the light. I had dropped a plug and put it in the cylinder. well that plug didn't fire. I put the timing light on and it didn't light up, so if a plug is missing it should show a disruption in the light. Now I did notice that you have green strip modules.
    Those are junk. I have replaced both of mine. good luck with your find, sometimes the rotax can be a real pain to find a problem.
    steve
    slyfox
    model IV 1200-flying
    912uls
    IVO medium in-flight
    RV7A-flying
    IO-360
    constant speed prop

  7. #7
    Birdseyeview's Avatar
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    Default Re: Engine missing on "L" ignition switch setting

    John, Thanks again for the tip to check the spark plugs. That should have been the first thing I changed out but when I did replace them the problem went away. The engine is purring like a kitten again. I haven't checked the old plugs yet for resistance but I'll bet one or more is bad. When the engine was first run I had a problem with fiberglass residue in the wing tanks due to some brief exposure to ethanol gas and it gunked up the carbs and the insides of the manifolds. At the time I cleaned out the carbs, and manifold/head passageways but I didn't change the plugs. I'm sure the residue took its toll on the plugs and probably explains why they didn't make it to their normal 100 hour change interval.

    Steve, Thanks for your suggestion to strobe all the plugs. Also, based on all the past negative discussion about the 912 ignition modules on this forum I suspect your complaint about them is justified. I got fortunate this time and didn't have to replace either of my vintage modules. When the time comes I'll probably replace them with one of the cheaper aftermarket versions with the soft start feature.
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  8. #8
    Senior Member Slyfox's Avatar
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    Default Re: Engine missing on "L" ignition switch setting

    those green stripe modules will stop working when you start up. generally cold temps. than start working after they get above like 2000 rpm maybe 3000. take care
    steve
    slyfox
    model IV 1200-flying
    912uls
    IVO medium in-flight
    RV7A-flying
    IO-360
    constant speed prop

  9. #9
    Senior Member
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    Default Re: Engine missing on "L" ignition switch setting

    I've bought those aftermarket ignition modules that are selling for $245 from the guy in Wisconsin or Illinois. They have built in soft start module. Only ran one for testing purposes so far, and it was fine. The other one will go on my ULS so I can get the soft start. That is, if I ever finish that Kitfox..... not much progress this summer. JImChuk

  10. #10
    Super Moderator Av8r3400's Avatar
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    Default Re: Engine missing on "L" ignition switch setting

    When "testing for spark" do not *EVER* unplug a spark plug and test the ignition with an automotive type spark checker or pull the plug and hold it to a ground, this is a sure way to fry a module.

    Spark plugs are cheap. It's also been recommended to me by Hal Stockman to gap at .5mm (.020") to .6mm (.024") for best starting and running performance. (OEM is .7-.8mm)
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
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