Ignition issue

RV12, 400hrs. , Exp/amateur built, by me, IRMT

Started with a bad mag check, “A”module, rough and -200 rpm.  Carbs sent for 200 hr maintenance, reinstalled/sync…no change.  New plugs, air filters…no change.  Modules sent to Lockwood, tested good, no change.  When i switch module horizontal plugs putting A to B, and B to A, the exact problem changes to module B.  When I switch them back it goes back to “A”.  The mag check done at 3500 rpm is particularly bad.  The plugs when I changed them were dry black sooty, all 8.  The roughness is worse at 3500and actually clears after running it at 4-4500 rpm.  I do have a long taxi and I have heard the issue of black plugs from long taxi/idles, but in 400 hrs I have never had even a hint of a bad mag check…even with several long waits at Oshkosh!

I suspect that module A has a problem in the lower rpm ranges that didn’t show on Lockwoods testing.

is that possible?