Most carburettors operate on the principle of vacuum. As the piston moves down in the cylinder, vacuum is created in the intake manifold. Carburettors are attached at the inlet of the manifold. They contain one or more passageways called barrels that air passes through.
One system used by the carburettor is called the choke. The choke is employed during cold starts. It consists of a choke plate located at the top of a barrel that closes when the engine is cold, shutting off air flow from the top of the component. This increases the vacuum in the barrel and results in fuel being pulled through various ports.
During idle conditions the throttle plate at the end of the barrel is closed. This shuts off the barrel from the intake manifold. An idle port located just beneath the throttle's closed position allows the manifold's high idle vacuum to draw fuel through.
The idle system contains another port above the main idle port called the off-idle port. As the throttle plate opens to transition from idle to normal speed, more fuel is needed than can escape the idle port or pass by the partly-opened throttle plate. The off-idle port provides additional fuel at this time.
When the transition from idle to cruising speed is abrupt, an extra system comes into play to allow more fuel into the cylinder. The accelerator-pump system provides additional fuel by way of a small fuel pump in the carburettor. The pump is connected to the throttle by a linkage.
Once normal load condition and cruising speed are attained, the carburettor’s main metering system takes charge. At this point, the piston's movement is creating a sufficient amount of vacuum in the barrel.
During heavy load conditions the power system is employed. There are two different types of power systems used. One involves bypassing the main jet, or the hole in the fuel bowl through which fuel generally enters the air stream, and redirecting fuel through a larger jet. Another common design uses a larger main jet that is partially blocked by metering rods. When the power system is needed, the rods are removed to increase the jet's size.
Performance carburettors offer improvements on the standard component. Holley, for example, is well known for making carburettors that are easily adjustable. They provide adjustable fuel level in the bowl, idle air-and-fuel mixture, main jet size and accelerator-pump nozzle size.
Many performance carburettors include four barrels to deliver a much greater amount of air and fuel to the engine than a single- or double-barrel model. Two of these barrels comprise the secondary system. This system has its own throttle plates and is usually employed when high speeds or loads are encountered. How quickly these throttle plates open is sometimes adjustable.