From you symptoms, it is hard to say if it is lean or rich... It looks like the choke butterfly is snapping closed when cold and not running; is this correct? Is the choke butterfly popping open a bit when the engine fires? It should open up a bit as airflow gets established; this is the choke pull off function. If not, try turning the choke adjustment (the round black thing on the passenger side) 1/8 to 1/4 turn CW to lighten up the choke action a bit.
With cold weather, it may be a bit hard to get the choke and fist idle set right, especially with a strong cam, so it may be easier to just keep lightly pumping the throttle and holding yourself at a fast idle setting to get it warmed up, and then set the throttle higher for break-in. For anyone not used not used to carbs (and you may indeed be), this is a learned technique.
Also, look down the secondary throat and see if the back edge of the secondary throttle plates have just a tiny crack between the rear edge and the throat.