Just to add more anecdotal evidence to the above, the 1978 VW Derby that was my first car also suffered badly with carb ice on winter mornings, until the exhaust warmed up enough to provide sufficient heat... it was quite interesting approaching traffic lights with one hand steering, one hand changing gears, and one hand adjusting the choke, whilst simaltaneously having my left foot on the clutch, my right foot on the brake and my other right foot on the throttle to keep the thing running...
Genghis - I don't know anything about Rotax engines but my 1967 Mk II Spitfire has what I would call 'coolant-loop' carb heat - the intake manifold casting incorporates a channel for hot coolant to pass through. See part number 307625 on this schematic:
It's never suffered with carb heat, although it has innumerable other problems...
cbl.