Q: What does a carburetor do?

In the simplest of terms, carburetors provide atomized fuel to the engine for combustion. By using air pressure and a series of mechanical control devices, carburetors can take liquid fuel from the gas tank and mix it with air as it enters the engine through the intake manifold. Liquid fuel needs to be atomized and finally vaporized to allow for proper burning and combustion in the internal combustion engine. In order for the liquid fuel to change states to a vapor, carburetors were used for decades until the advent of fuel injection. So how exactly does the carburetor perform this task? Think of a carburetor as a control tunnel consisting of several circuits internally. We start with the fuel supply, which is a small bowl area that holds fuel readily for use similar to the tank on the back of a toilet keeping the water on hand. In fact, the float which controls the fuel level operates similar to the float that controls the fill level of water in a toilet tank. The carburetor has a cold start element called the choke that allows for more fuel on startup because a cold engine requires a richer fuel mixture. By restricting air on startup, the fuel mixture becomes richer.

The throttle plates are the main air control element for the carburetor. There is an idle mixture circuit, a high speed fuel circuit, and an accelerator pump to provide additional fuel when depressing the accelerator pedal on acceleration. Each circuit has a specific function to allow for the proper air-fuel ratio, which is 14.7:1. This ratio states that 14.7 parts of air are mixed with one part of fuel for optimal burning. In professional terms this is call the stoichiometric ratio. Proper operation requires clean air entering the top of the carburetor, called the air horn, and then the air speeding through a series of tunnel type structures called venturies. These restrictions cause the air to speed up and allow for a pressure drop which makes the fuel come out of the jets or fuel orifices and mix with the incoming air charge. Air pressure is necessary for this process to work. When you drink from a straw you assume you are sucking the liquid up through the straw. However, the liquid is actually being pushed up through the straw by the air pressure on the surface of the liquid. The pressure on the end of the straw is lower, allowing the greater pressure to force the liquid up the straw. In the same manner, fuel is forced through the carburetor circuits by the air pressure on the fuel in the fuel bowl. Proper adjustment as well as keeping the passages clean of dirt allow for the carburetor to perform these tasks. The fuel pressure on these systems is usually only about 5 psi (pounds per square inch) since we only need to keep the fuel supply at the carburetor bowl filled.

Modern vehicles use fuel injection, which is more precise and runs at much higher pressures. However, air-fuel measurement is done in a similar fashion to carburetors. A MAF, or mass air flow sensor, needs to count or be exposed to the air entering the intake system in order for the computer to add the correct amount of fuel. Just like the old carburetor systems, if air enters through a cracked line or intake with a broken or misaligned gasket, the system will run lean on fuel. The carburetor will only allow fuel for the quantity of air entering through the air horn. Carburetors are on older vehicles and are still used on small engines like lawn equipment, 4 wheelers, chain saws, and alike. If the carburetor fails and needs service or replacement, seek service from a professional technician who is familiar with the proper adjustment procedures.