1993 b2200 : fast idle in cold weather
 

I have a 1993 Mazda B-2200 w/carburetor & 5-speed transmission. When the outside temperature is below 46-48 degrees , the engine idles at about 2200 RPMs with the choke completely open.( There is a noticeably difference in fuel consumption and power when this takes place).The RPMs will reduce to normal after about 20 miles.
I had this same problem last winter, but as the temperature rose in spring, the problem stopped. Now it is happening again.(I replaced the coolant sensor, but nothing changed). Any ideas?