Engine runs rough on one line

Hello everyone,

I have a bit a problem. I use a Rotax 912 ULS from 2020. There are issues with rough running in the lower speed range (2,500–3,500 rpm, sometimes varying). There is no problem in the higher speed range.

Carburettor synchronisation and an overhaul haven't really helped. I've now come across the following: When I used my magnet check with line B off and vice versa, line A runs smoothly and line B runs rough.

With magnet check A, the speed is reduced by approx. 100 rpm; with line B, it is approx. 120-150 rpm.

I would now look at the following: First, I would replace the spark plugs, then the spark plug connectors. If that didn't help, I would swap the two ignition electronics boxes. 

What do you think? Has anyone had a similar problem? Or can you think of anything else that might be causing the problem?

Thank you

Konrad