912 ULS Power Reduction

If they are vented into the slipstream (versus just into the relatively static air inside the engine cowl), that is NOT how Rotax recommends they be vented.  The carb uses that "vent line" to basically measure the atmospheric pressure, and adjust the mixture accordingly.  Having the vent lines in the slipstream results in significantly lower pressure, fooling the carb into adjusting the mixture far too lean for the actual altitude.  Excessively lean carburetor settings can cause all sorts of nasty things to happen to an engine, including detonation.

Those vent lines are supposed to either be tucked in behind the carburetor bowl clips, or if you have the Rotax airbox, there are dedicated ports on the airbox for them to attach to.  

The idle mixture screws are supposed to be opened one and a half turns from the closed position.

"Atmosphere" and " Slipstream" are two totally different animals!

The carb Vent lines should terminate near the Carb Air Filter and NOT to the Slipstream beyond the Fire Wall.
The vent lines ideally should be at the same pressure as the intake throat of the Carbs.
Some installations connect the vent lines to a fitting connecting to the internal pressure of the Air filter.

That's the way I do mine Bill. I like it because if the vent lines ever start puking raw fuel it goes into the carburetor instead of down on the hot exhaust manifold. The excess fuel will then cause the engine to run rough so you know right away something is wrong.