The carburetor is classified by the number of pathways that it uses to allow air to flow into the engine.  These pathways are called venturies, but are more commonly known as barrels.  A small engine may have a one or two barrel carburetor, while larger engines utilize a four barrel carburetor.  For high-performance applications an engine may use multiple carburetors that provide each engine cylinder with its own barrel or venturi.

The carburetor has many jobs to perform. It is supplied with fuel from the fuel system and holds a small amount of fuel in a compartment called the bowl.  It must measure, and control the airflow into the engine while adding the appropriate amount of fuel for the conditions at hand.  To perform these tasks it employs several systems that work in symphony during the five different types of engine operation.

Airflow control:

Each barrel or Venturi is sealed at its base by a movable air valve called a throttle plate.  This throttle plate is connected to the accelerator pedal.  Movement of the pedal opens the valve and allows air to flow into the engine.

The Main Circuit:

The main circuit comprises a pathway from the fuel bowl to the center of each barrel called the main fuel nozzle.  This pathway supplies the fuel for all operating conditions of the engine except for warm engine idle.

The Idle Circuit:

During warm engine idle the airflow through the barrel is not high enough to draw fuel through the main circuit.  The idle circuit is an additional pathway from the fuel bowl to a low pressure area below throttle plates.  It supplies fuel during warm idle only when the throttle plates are closed.

Cold Start:

During a cold start, the engine requires a richer air fuel mixture (more fuel per unit volume of air).  This is accomplished with the use of a choke plate.  The choke plate and is another movable valve is mounted at the top of the carburetor. When the choke plate is close to the mixture is richened.

Accelerator Pump Circuit:

A small pump is mounted and another pathway between the fuel bowl and barrel. The pump is connected to, and moves with the throttle plates to add fuel only with the sudden throttle plate movement.

Power Valve Circuit:

The power valve circuit adds additional fuel when the engine is under high load.