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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?

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