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I have a 2010 912 ULS that several mechanics have improperly diagnosed. I brought it in to the first mechanic because of a misfire. I feels very bad and happens on takeoff once it's warmed up. I've tried to duplicate it during run up on the ground and once we've taken off and it's started "skipping" / misfiring, I can get it to kind of do it on the ground. It's a misfire you can feel in your feet and the rpm drops maybe 100-200 rpm from 4,000. First mechanic did no diagnostics and did / recommended gearbox check, 5 year rubber, Carb rebuilds, spark plugs / oil change, boots, and after all that the misfire had gone until after I flew it home. The next time I went to fly it there was the misfire again once we had taken off. I don't think it's the gearbox slipping because the engine runs smooth if it's not misfiring. It's not a consistent rough running like when carbs aren't synced, or the gearbox is bad. I did my flight training in Rotax 912 planes and so I've felt the difference between the different motors and can tell the various symptoms of carb issues or carb balance etc. This feels like a sudden jolt I feel in the rudder pedals, like what a misfire feels like in a car. The only thing that hasn't been replaced at this point is the ignition coils.... So, I saw a video the other day of similar symptoms, and he had checked for resistance on the coils to see if they were between 16-18 ohms, and he used a heat gun to heat up the coils to see if the windings would open under heat, and he was able to find a bad coil that way. I've checked resistance on all the coils and they all checked out, even when hot. Then a second mechanic recommended replacing the modules. He Replaced modules, next takeoff - same misfire. HOWEVER... the first lead that I may have found on the actual problem happened the other day when checked resistance again. I was tracing the wires from each of the coils to see which plugs they went to, and when I lightly (very lightly) pulled on one of them, it slid out from the coil and the two wires were exposed. This made me think potentially there was a loose connection because it came off so easy. Can I just strip back the spark plug wire and twist it back into the coil? The mechanic said he could do that and that potentially was the issue. I figured I'd check with everyone else first to see if they've seen a similar issue / symptoms with a misfire due to a bad ignition coil or multiple failing coils. At this point I'm leaning towards replacing all 4 coils only because of the sunk cost I've already invested. Any two cents welcomed

8554_1_37A11F84-392C-451F-AC8C-E78B8D81B457.jpeg (You do not have access to download this file.)
  • Re: 912 ULS Engine Misfire

    by » one month ago


    Looks like you found the problem. I wouldn't replace the coil if they tested good. Keep eye on the plug wires, especially where they pass under the intake manifold as any contact there can cause shorting out.


  • Re: 912 ULS Engine Misfire

    by » one month ago


    Ther coils very rarely ever go bad. , but rarely at the coil unless some kind of maintenace has been hard on thsoe connections. 


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


  • Re: 912 ULS Engine Misfire

    by » one month ago


    You can easily cut a short length off the HT leads and just screw them back into the coil, they certainly shouldn't just pull out like in your photo. There is also meant to be a length of heatshrink covering the nose on the coil. I cannot remember whether it is meant to be adhesive lined.

    What I would say is that a failure of a single HT lead is unlikely to lead to an RPM drop as high as you report. That level of drop is more likely due to a complete ignition failure on one side. Have you tried switching off a single ignition when the misfire starts to see if you can isolate it? Have you also checked the resistance of the NGK boots fitted to the plugs? I cannot remember the resistance, I think that they were somewhere in the range 5-10k ohms. You can measure from one plug boot to its paired one through the coil, this will save you from taking the boots off the HT leads to make the measurements.

    I would check things like the generator coils that supply the ignition modules. There are a couple of ground connections under the flywheel and there have been problems with the red supply leads on some engines. A loss of either supply will give you a fairly large RPM drop.

    Good luck!


  • Re: 912 ULS Engine Misfire

    by » one month ago


    Thanks for the info! I will check those things and I'm taking it to the mechanic on Tuesday. They are going to look into the items you mentioned and I haven't tried isolating the misfire yet since it normally happens on takeoff (not the best time to switch off either side). I will try doing that on the ground to see if I can figure out which side is bad. I really appreciate the feedback and think that is more likely the cause as well based on the symptoms and your feedback. Will keep you posted! Thank you!


  • Re: 912 ULS Engine Misfire

    by » 6 weeks ago


    The high tension wires from the cold to the spark plug are a copper stranded type.  Be sure that you cut back the wire about 3 to 4 mm (1.8 inch at least) to get into a clean part of the wire.  Use a dielectric grease on the wire end also, do not install it dry.  This will help prevent corrosion that may occur over time.  The sleeve on the wire is for electrical suppression, it is simply a vinyl pull on peace and is not glued or stuck to the wire.  It is there to help reduce interference with radios and the like.  Let us know if this works for you. 

    Cheers


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