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We've never, nor now, have anything but perfect normal running the mag test at the recommended approx 3600 RPM on our 20 year old 1100 hour Rotax 912UL. Either mag turned off drops the RPM about 150.

 

But for some reason we tried running a mag test at low (1900) RPM.

Turning off the right mag and it drops about 100 RPM.

Turn off the LEFT mag and engine dies immediately.

Playing with it more reveals:

If above about 2500 RPM mag tests constantly behave normally. Get below about 2200 and turn of left mag and engine dies.  Thus is true regardless of whether engine is warm, hot, or cold.

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FYI FWIW: Checked all plug leads ... all seem snugly on their plugs.  Plugs replaced with new properly gapped plugs about 10 hours ago. Battery is pretty new and well charged. 

We do tend to have a tad of trouble starting .... which would be consistent with a scenario we infer exists.... which is that at _low_ RPM (as is the case when starting) that only one plug in each cylinder is firing.

 

Any ideas? Diagnosis? Cure?  Rotax doesn't have traditional magnetos.  Any idea what the module/parts functioning as if they are mags are? Their cost? Their access?

We're not too worried that this is a safety issue as at normal flying RPM range we appear to have two  perfectly functioning ignition systems. But.....

And BTW: Which plugs (upper or lower) does the right mag feed?

  • Re: Mag test fails ... but ONLY at low RPM?

    by » 11 months ago


    Sounds like a failing ignition module, just swap them and see if the problem moves with it.


    Thank you said by: Al C

  • Re: Mag test fails ... but ONLY at low RPM?

    by » 11 months ago


    Thanks!

    Off hand (I'm not at the airport looking under the cowl) does that swap sound like a tough, easy, or medium job? 


  • Re: Mag test fails ... but ONLY at low RPM?

    by » 11 months ago


    It should be just a matter of disconnecting and reconnecting the plugs/connectors.


    Thank you said by: Al C

  • Re: Mag test fails ... but ONLY at low RPM?

    by » 11 months ago


    If the connector swap doesn't reveal anything;  Consider that a trigger coil may be loose and moved.

    Rev it up to 3000rpm, turn OFF the good Mag, and then Slooowly reduce the throttle.

    Does it die suddenly and peacefully or is there a second or two of Death-Rattling before it succumbs?

     

    At the correct gap of ~0.4mm the ignition should fire down to ~200 rpm.

    If it moved a mm, 2200 rpm would not be out of expectation.

    Checking/Adjusting the Trigger Coils might require removing the engine to get access depending on your aircraft.  Not Fun!

    - - -

    If, IF!, your "Right" Mag is connected to the "B" ignition module, then it controls the 1&2 Top and 3&4 Bottom plugs.

    You need to spend an evening with the Heavy Maintenance Manual, Section 74-00-00.


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


  • Re: Mag test fails ... but ONLY at low RPM?

    by » 11 months ago


    Thanks Garrett!

    You wrote ..

    "It should be just a matter of disconnecting and reconnecting the plugs/connectors."

    I was imagining, or at least hoping, that, as you say, we could just swap the connectors.

    Haven't yet been out there.

    Al


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