It may be normal if it is that cold. How long ago did you replace the spark plug? That would be a good place to start. It could also be a partialy cloged pilot jet (starter jet) in the carb, that is pretty common too. Are you running a fuel filter? Replacing the plug with a hotter one will always increase the risk of burning a hole in the piston crown. I would try a new plug and possibly carb cleaning first.
THE 80's RAGE! Currently owns:
81, 85 Yamaha Tri Moto 125
83 Yamaha Tri Moto 200 x2
82,83 Yamaha Tri Moto 175 project
85 Yamaha Tri Z 250
84 Kawasaki Tecate 250
84 Honda ATC 70
85 Honda ATC 350X
84 Honda ATC 250R
(2) 84 Honda ATC 200x
82, 83 Honda ATC 185s
81 ATC 200
Lots of other parts and bikes