I think it has somthing to do with the size of carbs used.
Round slides arnt used anymore but they had one major advantage. A primer instead of a choke. If your pump or carb was frozen you just warmed it up by pumping gas in with the primer and eventually the engine took and ran.
Actually, the reason you see the heated carbs on the flat slides is do to the icing problems inherent to flat slide carbs. Round slide carbs are less susceptible to icing, and hence, not necessary. The reason they are seen on the twins more than the triples is due to their bore sizes. The larger the bore, the more air, and hence the more likely the carb is to icing problems. They add nothing in terms of performance, just minimizes the icing problems. Ski-doo had a lot of icing problems on their big twins in 2001 when they went to the flat slides from the round slides, and for 2002, they added heated carbs. You are suppossed to use them when the temps go below freezing. (The owners manual actually says to turn them off when operating above 43 degrees farenheit-like we do that all that often)
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