Supplementary daft question: How does a Rotax engine adjust the fuel/air mix with an increase in altitude?
Rotax 4 strokes (912) use Bing CD carburettors. CD standing for Constant Depression. These use the butterfly valve to signal demand for fuel/air mixture, but the actual metering is done by a slider & needle which is raised by the vacuum applied from the butterfly valve. The mixture at any given setting is determined by the profile and height of the needle, and the size of jet it slides into. At low altitudes the slide would lift all the way up if the butterfly was fully open. As the air gets less dense higher altitudes the slider would progressively open less and less, so the available power decreases but the air/fuel ratio stays the same. Stromberg and SU carburettors work on the same priciple.