I drive my ktm 125 all year round. Assuming good fuel, compression, plug and a quality oil at 40/1, you need to richen it up. Try raising the float level 2-3mm. Use a hose to blow air thru the float valve to see where the level is now (mark it!) and bend the tab to raise the level. This will richen the mixture in all the range but is what you want for cold weather.