With the air temprature and the interior surfaces of the manifold both cold the fuel will fall out of suspension at low throttle openings and will puddle on the floor of the manifold. Not until a small amount of heat is transfered into the manifold causing the fuel to resist puddling will it begin to idle acceptably. Manifolds without an exhaust crossover and single plane type manifolds are the worst. A choke will reduce the amount of air that can be pulled through the carburetor and causes additional fuel to be pulled, temporarily making up for what is puddling. Once some heat is developed and all the fuel is being atomized the air fuel ratio has to be leaned out to keep it running (by relieving the choke). If you have no choke, additional fuel is added by blipping the throttle pedal.
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