[AZKen]

The choke is part of the "cold start circuit". We not only have a cold start we have a possible empty carb bowl and lines. Closing the choke is how you start a cold/empty motor. The accelerator pump delivers the rich shot as described by me and Fetch. There is not much vacuum/venturi effect at that point.

The needle valves are for idle circuit dribble and main circuit. The main circuit takes over as you press throttle/open the throttle plates and create the venturi effect. The main jets and the rest of the main circuit does add some air to the gas to help in the atomization (air/fuel mixture at/in the venturi). I am not an expert but that is how I would describe it in general.

The richness and leanness is controlled by the valves and jets as you say. WE are ONLY talking about the cold/dry, after new pump, start subject of this post. Not the general running after warm up and on the road performance tuning.