Many people are coming in complaining that their motors are running rough in the cold weather. This is the result of the heat riser being plugged. If you have a stock carburetor, this can be caused by icing that takes place under the carburetor. Feel the manifold under the carburetor after you have been driving for awhile, if it is cold to the touch, or if you see frost around it, this is your problem.
What does icing have to do with the performance of my motor? Well, this is what happens: The gas and air are going down the intake at a high rate of speed creating a wind-chill, causing the sides of the manifold to freeze. Once it freezes, it will draw the gas away from the air and ball it up on the sides of the manifold. It eventually will run down the side. Now raw gas does not burn well, it has to be the right mixture to run right.
To correct the problem: If you are running a header, more than likely the heat riser stand on the header has not been drilled into the exhaust pipe. The header will have to be removed in order to drill these. Now if they've already been drilled or if you are running a stock muffler, then the heat riser tube is probably plugged. It is easier to clean this tube out if it's off the car, but not necessary. If you are going to clean it when it's still on the car, you must remove the exhaust. To clean it, you need a 16" piece of clutch cable, a drill and a thick pair of gloves. Spin the cable into the drill so the cable wraps up as it spins in the manifold. Hold the drill with one hand and guide the cable with the other. Go slowly so the cable doesn't whip around and hit you. NEVER REVERSE THE DRILL, if you do the cable will get stuck, you will never get it out and have to throw the intake manifold away. Repeat this procedure on both sides until you can blow air freely through the heat riser tube. This should solve your problem.