IMO this is one of the WORST problems with non-pumper carbs. The TW's "constant velocity" (CV) carbs need air rushing through the venturi for ANY fuel to be sucked into the carb. At that point there is already too much air in the chamber and it takes a lot of cranking to get enough fuel in.
CV carbs try to overcome this problem with a "fuel enrichment" circuit (the TW's "choke"). A plunger that squirts fuel directly into the combustion chamber would have been a better fix, IMO.
So, keep some fuel additive in your gas (Startron is my favorite but Seafoam is hard to beat). Store the bike in as dry a place as possible, especially during the winter. Make sure the battery is in perfect condition, The starter needs to spin the motor as fast as possible to make enough suction.