Your car’s carburetor (if it has one, most recent cars are FUEL INJECTED) supplies the proper mixture of fuel and air to your engine. A defective carburetor can cause smelly exhaust, smoking, hard starting, and poor gas mileage. A carb works by the VENTURI PRINCIPLE: which basically means that if you pass air across a hole, a vacuum will be created in that hole. A carburetor has many carefully positioned holes which the air sucked in by the engine flows past. These holes are connected to the CARBURETOR BOWL, which is kept full of fuel by a FLOAT AND NEEDLE VALVE. This valve and float shut off the pressurized fuel sent from the tank by the fuel pump. It works about the same way as a toilet tank valve. How much air flows by the venturi(s) is determined by the throttle plate, which restricts the airflow through the carb. At wide open throttle, all possible air is admitted throught he carb, and maximum power is gotten from the engine.When an engine is cold it needs a richer (more fuel to air mix than usual) mixture to run correctly. Carburetors have a device called a CHOKE to accomplish this. The choke is like he throttle, except it is on the top rather than the bottom of the carb, and it closes when cold and opens when warm. When the choke is closed, it restricts the air entering the carb at the top, while the throttle still allows the same amount of air to go through the bottom. This means the same amount of fuel is sucked, but less air: so the mix is richer. FEEDBACK CARBS The last of the carburetors were computer controlled. The American cars mostly used a jet in the carb bowl which was “pulsed” by the computer, opening and closing 10 times per second. The computer would keep the jet open a longer or shorter period of time to “fine tune” the fuel/air mixture. Asian cars sometimes used a vacuum line connected to the venturi itself. A computer controlled module would “bleed” a small amount of air to the venturi to lean out the mix and adjust it. Throttle Body Fuel Injection Many people see throttle body fuel injection units and think it is some kind of carburetor. It is not. It just has a single (or two or more) large injectors which are above the throttle plate instead of inside the intake manifold. There is no fuel bowl, float, venturi, or choke. News source: Econofix