I run mine a little richer in really cold weather. the air is denser and it needs more gas to get to the right stoichiometric ratio. your idle may need bumped higher as well but my idle worked ok just by turning the pilot screw on the carb to a slightly richer setting. not much. there is also the problem of the cold air causing condensation inside the carb. the gas also does not want to atomise as much because of the cold. it tends to vaporise better in more moderate temps. typical carb problems. some carbs have a heater system to counter the condensation problems. if you don't stall at start-up it is because your choke system is getting more gas to the carb in relation to the air. when the choke system is off (when it is warm) you get less gas. you need to add more gas to the air-fuel mixture. Good Luck!
