What Is a Carburetor?

State of Speed Basics – The Manly Science of Automotive Knowledge

There are three things necessary for an internal combustion engine to operate – Fuel, air, and a source of ignition. For most of automotive history, one piece of hardware controlled two of the three required elements: the carburetor.

At its most basic, a carburetor is any device that combines fuel and air into a mixture that will support combustion. In Perfect Chemistry Land, gasoline wants 14.7 parts air to each part fuel for a complete burn, but because engines don’t visit Perfect Chemistry Land very often, a carburetor has to be able to deliver air/fuel mixtures that are both leaner (more air) and richer (more fuel) than this perfect “stoichiometric” 14.7:1 ratio, depending on many different factors.

Sometimes the ratio will need to change for better fuel economy, or for maximum power. It will have to be different when the engine is cold than it is when the engine is up to its normal operating temperature. And it will even need to change moment to moment as the throttle (the “butterfly” valve connected to the accelerator pedal that controls how much air passes through the carburetor and into the engine) changes position.

In order to do that smoothly and effectively, carburetors evolved from simple devices that vaporized fuel into what are essentially sophisticated analog computers. The main data inputs are the manifold vacuum and throttle position, which represent how much air the engine is trying to draw in, and how much air the driver is allowing it to have, respectively. Based on these two primary factors, a complex series of air passages, calibrated orifices called ‘jets’ or tapered metering rods, and any number of other clever mechanical devices controlling the flow of air and fuel work together to deliver the correct mixture to the engine.

Carburetors fell out of favor for factory vehicles in the late 1980s as electronic fuel injection became available (and less expensive), with the last carbureted vehicles sold in the US bowing out in the early ’90s. Manufacturers found that EFI made it easier to comply with tighter emissions requirements, but in the world of high performance, carburetors still enjoy a lot of popularity for both street and race vehicles.

The Holley 4150-style carburetor is easily the most well-known performance carb and can be found on countless different factory muscle cars and race vehicles.

Edelbrock’s AVS carburetors are an update of another classic 4-barrel carb design. This polished show-quality model features an electric choke – the black cylinder on the left side of the carburetor contains a mechanism that automatically opens and closes the choke based on the electrical signal from a sensor installed in the engine’s cooling system.

Holley’s ‘Dominator’ 4500-series carburetor is the big brother to the classic 4150, designed for engines that need more airflow than a smaller carburetor can provide.

Carburetor Terms You Should Know

The appeal of a carburetor to a gearhead is that all the necessary tuning can be done with a selection of simple parts and a few hand tools – no laptop (and no electricity, period!) is required to make adjustments. Racers will typically take careful notes of how an engine performed under specific air temperature, barometric pressure, and humidity conditions with certain carburetor settings and parts in order to make it easier to reproduce the correct tune in the future and predict how changes will affect engine power and responsiveness. 

While carburetors have been around for more than a century, they still have a prominent place in high-performance engine tuning and will be around for many years to come in both racing applications and under the hood of classic cars from around the globe. 

Carburetor Glossery

• Choke: A movable device at the inlet of a carburetor that is designed to restrict airflow and richen the air/fuel mixture for better engine operation while cold. Many racing carburetors have no choke, as it can cause a restriction to airflow even when deactivated. Engines equipped with chokeless carbs are harder to start and require constant attention to the throttle to keep them running until they reach operating temperature
• Side-Draft/Down-Draft: Depending on the layout of the intake manifold, carburetors may be designed to flow air and fuel horizontally (side-draft) or vertically (down-draft). Side-draft carburetors are common on classic Japanese and European cars, while American V8 cars are typically down-draft.
• 2-Barrel/4-Barrel: Muscle car carburetors will often be referred to by the number of “barrels” or venturi intakes the carburetor has. Often abbreviated as “2/4bbl” or “2v/4v”, 2-barrel carburetors were most often found in lower-performance applications, while high horsepower engines featured 4-barrel carburetors, or even multiple 2- and 4-barrel carbs.
• Vacuum/Mechanical Secondary: In a 4-barrel carburetor, the engine normally draws air from only two of the four venturis. This allows more precise fuel metering than if all four were in operation at all times. In a vacuum secondary carb, the main throttle blades are controlled directly by a mechanical connection to the accelerator pedal, but the secondary blades on the other two barrels only open in response to a vacuum signal from the manifold that indicates the driver is at wide open throttle. A carb with mechanical secondaries has a mechanical linkage that progressively opens the second pair of throttle butterflies in response to the position of the accelerator pedal. Vacuum secondary carbs are considered more “streetable” and deliver better fuel economy than race-oriented mechanical secondary carbs.

This Holley “Tri-Power” intake setup combines three two-barrel carburetors on a single V8 intake manifold for a cool vintage look and high performance to match. 

Many classic Japanese and European cars with inline 4 and 6 cylinder engines used the classic Weber DCOE side-draft carburetor. The side-draft design allows a very low hood line because the intake manifold and carburetors (often one per pair of cylinders in performance applications) don’t extend above the top of the engine. 

While you won’t need a laptop to tune a carburetor, you will need an assortment of different components like metering jets, power valves, accelerator pump squirters and cams, and other small parts. 

This Holley street carb has both a choke (the rectangular gold plate) for easier cold starts and vacuum secondary throttle blades (controlled by the round, silver vacuum diaphragm mechanism at the top.)