I found this it may be if some help;
-I have a bad mag drop, what do I do?
First determine if you have a carburetor problem or an ignition problem;
(WHAT?? Yes, the Rotax 9 series dual carb system can confuse the owner as a carb problem may show up as a high mag drop on one ignition system) Procedure:
Run the engine at 4000rpm (approx 1700 prop rpm) and perform a mag check. Record the numbers.
Slowly pull the choke out half way, the RPM should rise about 200 RPM.
Try the mag drop and record numbers.
If the problem is worse or better the carbs are suspect.
If there is no change the carbs are not suspect, you likely have an ignition