912 excessive RPM drop on lane A

I have a Rans S-6-ES Coyote II, the engine is a Rotax 912 ULS 2002.  Airframe and engine 177 hrs.my problem is 400 rpm loss when I shut off lane b

new spark plugs gap .024. New resistor caps. New wires. Pulled 6 and 4 pin plugs apart and took each individual wire out of its socket, checked for contact 

with the opposite wire. Lightly pulled on wires checking for broken wires.  Continuity test between trigger coil were wire is soldered and its opposite end at 6 

pin connector. Cleaned all grounds at intake manifold and at module bolt. Pulled coils out and visually checked wires. Ohm each coil between the two 

primary leads, results were t1-t2 15.47 b3-b4 15.48 t3-t4 15.46 b1-b2 15.15. I made an inline spark tester and with my wife in left seat after sunset tested for 

spark. this is where I’m at a loss.the drop occurs when I shut off lane b or bottom so lane a or top spark plugs are the problem. Cyl 1 and 2 to appear to be 

normal in that I shut off A I lose spark top spark plugs and B I lose bottom spark. On cyl. 3 and 4 its opposite. I shut off A and I lose spark on bottom cyl. 3

and 4. B I lose spark on top cyl.3 and 4.i have checked for crossed spark plug wires numerous times all is good. With both mags on all spark plugs fire, 

carbs are balanced, runs smooth, no power loss, full rpm range. No starting issues hot or cold. Any input from those guys and gals that never sleep at nights thinking about this stuff would greatly be appreciated