We find that the limiting factor on how low you can idle is the roughness of the engine at tickover. We are using a Rotax 912 and if you let it get below 1100 RPM or so you are afraid the engine is going to jump out of its mountings. While warming up, we usually increase the revs to 1700 or so and hold it on the brakes. No amount of carb balancing will seem to allow an idle of less than 12-1300 RPM with any chance of keeping the screwdriver in the adjusting screw slot.
We are a bit paranoid about engine stability as we found all 4 mounting brackets broken during an inspection last year. It has taken 7 months to get it all repaired and the engine is much smoother at low revs now.
What stable idle speed do others get down to on this engine?