My aircraft is powered by a 912S 100hp and I've done more than 1100 happy Rotax hours over 20 years. At fast cruise using about 75% power the fuel consumption is 18 l/hr. I've also been involved with servicing, maintenance and flying with these superb engines powering friends' aircraft.
These engines were designed specifically for mogas and 10% or more of ethanol is no problem, as seen in countries like Brazil where ethanol percentages are high. The big concerns about ethanol, in terms of adverse effects, are where it comes into contact with tanks and hoses which aren't designed to cope with it. Most automotive hoses are! Metal fuel pipes and metal tanks get corroded by it.
Of course ethanol does increase the risk of carburettor ice, but most Rotax 912 installations are in aircraft with engine cowls around the engine and consequently the carburettors are at the rear of the engine compartment in warm air. These days many/most Rotax 912 carburettors also have permanent hot water flow on the intake manifold side to keep ice out of the equation.
UL91 fuel in the UK is an excellent fuel for the Rotax 912 series, particularly as it has no lead. But generally it is almost as expensive as avgas. Avgas is not recommended for long-term use in these engines, although it can be freely mixed in any ratio with mogas and/or UL91. The problems with avgas are the need to change oil and filter twice as often, reports of thick lead deposits building in cylinder heads and on valves, plus the necessity to halve the intervals between propeller gearbox removals for servicing.
Last edited by Colibri49; 9th Sep 2017 at 09:31.