Extra fuel is also needed for accelerating. Suddenly opening the throttle increases the air flow, but fuel cannot flow from the discharge nozzle quickly enough to match it. An extra jet of fuel is needed. Depressing the pedal compresses a duration spring that exerts a force on the plunger of a small plunger pump. This pressurizes fuel below the plunger and closes off the inlet valve. Fuel flows through a bypass jet and enters the air stream from a discharge nozzle above the venturi. The duration spring extends the time for delivering the fuel.
Releasing the pedal lets the linkage move the plunger upwards. The bypass jet closes and the inlet valve opens, to let fuel refill the pump chamber from the float bowl.
When a cold engine is being started, little air flows through the venturi, and there is no heat to assist in vaporizing any fuel that is delivered. This makes the effective mixture of fuel and air too weak to be readily ignited by the spark plug. An excess of fuel must be supplied temporarily to ensure that the proportion of fuel that does vaporize will form an ignitable mixture. A choke valve is fitted to help. Closing the choke valve closes the carburetor intake. The pumping action of the pistons creates a low pressure area below the valve, even without venturi action. This low pressure causes fuel to flow from the discharge nozzle and from the idle and low-speed ports, and provides the rich mixture needed to start the cold engine.
The choke can be controlled manually by a cable that operates the valve.
Most are controlled automatically, so that the valve is closed when the engine is cold, and opens progressively as the engine warms up.
When the engine is warm, the fuel drawn into the manifold during starting vaporizes readily, and the engine can be started without the aid of a choke.
The choke should operate as briefly as possible. Overusing it produces rich mixtures that cause exhaust pollution, and increase fuel consumption.
Flooding a carburetor also produces rich mixtures. This can be caused by wear, or by dirt trapped in the needle and seat that causes the level in the float bowl to rise, and fuel to discharge from the nozzle - with little or no venturi action.
