912uls mag check

I would begin by suspecting the Spark Plugs.

It sounds like one of them is dead.

You may just find that one plug wire is not really connected.

Otherwise, remove the plugs and keep track of which location each came from.

If you find one that one looks significantly different from the others, that is the place to start.

Following Bill's advice, installed 4 ea. new plugs controlled by left mag, no change.  Switched mags, no change.  Bypassed keyed switch, no change.

Any other thoughts?

Before hanging the engine did you verify that the ignition coil trigger gaps were within spec?  On my 912ULS the spec is .012" - .016".   See the ROTAX 912 Heavy Maint Manual for details.

Check your EGTs during the run up. They should give you an accurate idea of what is happening. On a Lyc/Conti the failure of one mag will cause your EGTs to rise since the mixture combustion is not complete before the exhaust valve opens. I assume that it is the same on a Rotax.

If you have 3 normal EGTs and 1 reading very low, it means that plug is not firing and may result in a greater than normal mag drop