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Hi, all!

I've seen quite a few posts on rough running on one mag, but nothing jumps out at us yet as matching our issue. Help! :-)

We have a 2015 912ULS with about 550h on it. We're at about 75h in this 100h cycle with regular flying (although less recently because of weather and pandemic, I note), and suddenly we hear a small but distinct-to-us roughness around 2200rpm. A regular mag check at any speed above 2800rpm is perfect; we get <= 120rpm (digital gauges with 120rpm steps) drop on each with smooth running and she purrs at everything from 1560 to 5k -- except for that not-quite-right sound at 2200 or so. Trying a mag check there (yes, that's quite slow) shows drop but smooth on one and drop with rough on the other. There's definitely something hinky there.

We pulled the plugs and they were all (not just the fronts) pretty sooty (probably just from taxiing a lot and trying quite a few mag checks, but ...), so we swapped out the whole set, to no avail. We checked the carb sync and we're tighter than 1/2 psi at 2000, within 1/4 psi at 2500, and equal as far as we can see at 3000 and up. There are no nicks, kinks, crushes, or melts in the plug cables. I think we've eliminated the easy things :-(

Is there an easy way to check the plug boots & their attachment? One report noted that a bad build suddenly pulled loose after 1400(!!) hours. Other than that, it looks like we're down to taking apart the ignition harnessss and swapping things around ... which we've done in class, but only briefly and quite a while ago now. Reading through the ignition chapter of the MMH is not a little daunting :-) Not sure what's after (or before?) that, either.

Any suggestions, pointers, tips, advice?

 

Thanks in advance! 

  • Re: rough on one mag on 912ULS

    by » 3 months ago


    You can do a resistance check on all four coils through the spark plug caps, it should read about 15K ohms. Check the the top front, top back, bottom front, bottom back. The circuit you will be measuring goes from one plug cap/connector through the plug wire, the coil, plug wire and out the other cap. Some times when pulling plug caps off the spark plug they get pulled away from the wire and or corrosion builds up in the caps connection to the wire. The resistance check is a quick way of determining an electrical issue.


    Thank you said by: Get Inspired Flight

  • Re: rough on one mag on 912ULS

    by » 3 months ago


    The symptoms occurring at one particular frequency(RPM) indicates that something may be loose and shaking in sync at a particular harmonic frequency.

    The Spark plug connectors just unscrew. Lefty, Loosey!

    Try tightening each connector by one turn. Righty, Tighty.

    If you find one that is particularly easy to turn, keep turning until it hits bottom.

    If you have enough slack in the wiring, you can cut 1-2 wire diameters off of the spark plug cables to give the plug connectors a fresh bite on the wires.

     


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.


    Thank you said by: Get Inspired Flight

  • Re: rough on one mag on 912ULS

    by » 3 months ago


    Thanks, Garrett!  That sounds interesting and wonderfully concrete.  It speaks to me! :-)

     

    I'll go back to the MMH wiring diagram, but can you clarify for me which pairs to test?  I think you said 1 & 3 top and 1 & 3 bottom, right?  And then 2/4 the same way?

     

    Thanks again!


  • Re: rough on one mag on 912ULS

    by » 3 months ago


    Thanks, Bill!  I like the idea of a harmonic vibration. We just haven't pinnd down anything yet :-/

     

    I certainly can crank 'em down again.  I probably need to cut the zip ties, yes?  I'll go back to the MMH and check, but I'll take pointers on how hard to yank down the new ones if ya got 'em.

     

    Thanks again!


  • Re: rough on one mag on 912ULS

    by » 3 months ago


    It is as simple as the top front, top back and bottom front, bottom back. You can visually see/trace the plug wires to verify this.

    Check out dielectric grease for the connections.

    https://441py33rout1ptjxn2lupv31-wpengine.netdna-ssl.com/wp-content/uploads/tech_docs/tds/22058.pdf


    Thank you said by: Get Inspired Flight

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