I agree the carb flanges should be checked - I never understood why Rotax made them the way they did.

If the flanges were bad, as the engine heats up, it could cause expansion and air leakage around the flange causing more airflow than desired. This would cause a lean condition.

If that were the case, it would be nearly impossible to pneumatically sync the carbs and rough running would be noticeable anytime the engine is warmed up.

While I'm thinking about synchronization - I have always disagreed with the Rotax manual in this regard. They state to do mechanical sync 1st then pneumatic "if required". I believe pneumatic is ALWAYS necessary for a few reasons.

1. Mechanical only sets the idle stops.

2. I've never yet seen a multiple carb set up really synced unless done pneumatically. It can be dramatic!

3. You need to check the sync throughout the throttle range and that can't be accomplished mechanically.

A few of the most common pneumatic sync mis-steps are:

Not doing it with the engine FULLY warmed up to operating temperature.

Not checking the sync through the full throttle range.

Not checking the sync after carb work. It should always be done anytime a carb is removed and reinstalled.

Not checking the sync after carb adjustments. Any adjustment can (and usually will) affect the sync.

Greg