Except for the part thottle when starting, as thinksnow says, I've had no problems starting it when sled is hot and has been sitting 0-10 minutes. This is with carb heating on (do not have shutoff valve).
Have you checked float level? Stock setting is 13 mm. Have you checked chocke system for proper operation, that it will close completely on all carbs. I add some lube where the chocke rod goes into the carbs every now and then to ensure that they move out/in easily.
I go with thinksnow on installing a carb shutoff valve.