I had a 400 one time that was like that. If it would turn over it would start, but you had better leave the choke on full. After a few seconds it would start to stumble and then you pushed the choke in just till it would stop stumbling, then you played with it till you could push the choke in all the way. Then you set it at a fast idle and went to the house and had a cup of coffee and let it warm up before you used it.
The tank heater like was said is the best, It will not get it hot enough to move your gauge, but it will warm it up enough.