The Fuel Injection about Carburetor (2)
Cost is another major reason for choosing carburetors over fuel injection. Even though most cars today use fuel injection, carbs are still plentiful and relatively cheap. Used ones can be picked up at swap meets, via other car enthusiasts, speed shops or on the web. In addition, aftermarket manufacturers, including Barry Grant, Edelbrock and Holley, sell a huge range of brand new carburetors, designed for everything from mild street driving to all out drag and road racing.
Cost is another major reason for choosing carburetors over fuel injection. Even though most cars today use fuel injection, carbs are still plentiful and relatively cheap. Used ones can be picked up at swap meets, via other car enthusiasts, speed shops or on the web. In addition, aftermarket manufacturers, including Barry Grant, Edelbrock and Holley, sell a huge range of brand new carburetors, designed for everything from mild street driving to all out drag and road racing. In addition, there are numerous specialists out there to assist with tuning and rebuilding carburetors.
How Carburetors Work
In very basic terms, think of a carburetor as a length of pipe. At one end of the pipe is moveable flap that opens and closes – the throttle plate. This controls the amount of air entering the pipe. Further down, the pipe narrows inward (the venturi). On this section of the pipe there’s a small hole, housing a jet, which squirts fuel into the venturi. Automotive carburetors use three cycles or circuits to deliver fuel: idle, part and full throttle. Depending on how much the throttle plate is open, a certain amount of fuel is needed in proportion to the air rushing in order to allow the spark to ignite the mixture and fire the engine. The theoretical ideal or Stochimetric ratio for complete combustion is 14 parts air to 1 part of fuel (14:1); however, in practice, that is rarely achieved with carburetors.
A cold engine requires more fuel to operate than one that’s warmed up, since fuel is the more combustible of the two elements that need to be ignited by the spark. In the simplest of terms, the throttle plate, besides controlling the amount of air flowing into the carb, also controls the amount of vacuum in the venturi on both the top and bottom sides of the carb. On the bottom side, when the plate is closed, there’s a buildup of vacuum. By placing a secondary jet right below the throttle plate (the idle jet), the fuel can be drawn down the venturi more quickly and into the manifold and combustion chamber. This results in a richer air/fuel mixture, allowing the engine to start more easily when cold. (In addition, a manually-adjustable mixture screw can also alter the air/fuel ratio under certain operating conditions). In most automobiles, a secondary, moveable plate, called the choke plate, is mounted below the throttle plate. When the engine is cold, it fully blocks the flow of air through the venturi from the top side of the carb. As the engine warms up, the choke plate is pulled back, increasing airflow and reducing the flow of fuel from the idle jet. Chokes on automotive carburetors are either operated manually (often via a lever), or automatically, via an electrical wire and heating element that’s linked to the ignition system and fuel cut-off valve. As the electrical current warms up, the heating element expands, gradually opening the choke plate. The level of heat passing through the electrical current determines how much the choke is opened (more under warmer conditions, less under colder ones). Once the engine warms up, the carb switches from the idle circuit to the part throttle circuit, relying on the main jet to deliver the fuel. At full throttle, the main jet and air bleed jet (mounted at the top of an emulsion tube) provide fuel delivery. (The emulsion tube is a bit like a straw, which pre-mixes the air/fuel before it reaches the venturi and combustion chamber, to reduce the risk of detonation.)
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Olivia Tong is the freelance writer for e-commerce website tahiko.com and miparts.com offers the buyers around the world to find quality and discount auto parts. We try our best to aggregate leads in the business world, and let these leads benefit the entire business person.