The float chamber holds a quantity of fuel at atmospheric pressure ready for use. Its supply is refilled by a float driven valve; as the level drops the float drops too and opens an inlet which allows the fuel pump to deliver more fuel to the float chamber. The float rises with the replenished fuel level, closing off the inlet.
With some carbureted engines – such as those that drive power tools like brushcutters or chainsaws – a float chamber is unsuitable, as the engine needs to work even if it is upside down. In that case, a chamber with a flexible diaphragm on one side is used. Atmospheric pressure pushes the diaphragm inwards as the fuel is used. A needle valve connected to the diaphragm opens to allow the fuel to be replenished as the diaphragm moves inwards, pushing the diaphragm out again and maintaining consistent fuel pressure.
Venturi and throttle
In a carburetor, air passes through a pipe – in the form of a venturi – into the inlet manifold of the engine. A butterfly valve called the throttle, which is connected to the accelerator or gas pedal, rotates to restrict the airflow almost completely, or turns end-on to the airflow to allow free flow of air. This valve controls the amount of air/fuel mixture delivered to the engine through the venturi, and therefore also controls the engine's speed and power.
Idle and off-idle circuits
When the throttle valve is closed or nearly closed, the manifold vacuum created behind the throttle is sufficient to pull a small amount of fuel and air through small openings located after the butterfly valve. This is called the idle circuit and it enables the engine to keep running when there is no pressure on the accelerator. As the rotating valve moves forward to a slightly open throttle position, the vacuum is reduced, so additional small openings are revealed to compensate for this. This is the 'off-idle' circuit.
When the throttle is progressively opened, more and more air is allowed to flow through the pipe and into the engine. The idle and off-idle circuits cease to function because the manifold vacuum is now lowered, but as the airflow through the venturi increases, the Bernoulli effect, which lowers the pressure in the pipe as the velocity increases, sucks fuel into the airstream through a jet in the center of the throat.
If the throttle is opened wide very quickly, the idle circuit stops working immediately, but the main circuit does not become effective until the airflow has had time to build sufficiently. To bridge that gap in fuel flow, an accelerator pump delivers a squirt of fuel under low pressure to smooth the transition from idle circuit to main circuit.
Fuel ignites less readily when cold, and if the engine is also cold, then some fuel vapor can condense out of the air fuel mixture onto the intake manifold and cylinder walls. This makes the mixture leaner, so to compensate for this, a valve known as the 'choke' restricts the flow of air at the entrance to the carburetor, keeping the manifold pressure low even though the throttle valve has been opened. In this way, fuel is sucked into the incoming air through all the fuel circuits at once – idle, off-idle, and main. In some engines, instead of using a choke valve, an additional fuel circuit behind the throttle valve can enrich the air fuel mixture.