I have some experience with aftermarket carb systems and also think icing is the problem. You need 2 things for that -- are the intake manifold coolant tubes connected? -- in from the cover plate near the front of the head and returned to the line to the water pump (the heater hose attached to the metal tube under the manifold). The other thing you need is hot air from around the exhaust manifold. Older SAAB 99's and 900 models had a nice outlet with a built in metal strap to get attached to an exhaust runner. Some hot air tubing and foil covering part of the air filter. I haven't seen any aftermarket air filter housings that have provision for heated air but Pintos and Vegas used a Holley version of the Weber 32/36 with provision for that. -- Dave BTW - the electric choke on the Weber will come off to quickly in cold weather -- for the winter I install an ignition resistor to cut the voltage.