Ok this may be a simple question: Does running the motor richer/leaner have any discernable effect on engine performance? Is one better than the other for maximum acceleration/top end/etc for the rotax engine?
Like any combustible, there is a specific ratio of gasoline to oxygen that gives the optimum explosive mixture – in our case, the optimum mixture to enable the engine to deliver maximum power.
As air density changes, the volume of oxygen in the air changes so you need to change the main jet size in order to allow the carburettor to deliver the correct amount of fuel, to maintain the correct ratio and therefore maximise performance.
There is a lot more to correctly setting up the carburettor and getting the best performance from your engine, such as needle clip position, needle profile, float weight, float height etc.
Have a look at these articles, they may be of some help: