Sounds like the choke pull-off has a leak. It should pull the choke open slightly when the engine starts cold. As it warms up, the choke thermostat will gradually open the choke fully. To see if the pull-off is the problem, start the engine cold. Let it run for a bit until it screws up and starts smoking and then open the choke with your finger. If the smoke clears up and the engine smooths out (it may stall shortly after that due to the choke being too far open), that was it. The choke not opening enough is about the only thing that could could cause this problem only when it's cold.