How Does a Carburetor Work

How does a carburetor work? A carburetor is a complicated device that is manufactured to exacting standards. It is infinitely tuneable to the requirements of the engine it controls. Carburetors are thought to be interchangeable and produce the same results where fuel mixture and acceleration is concerned. This is only partly true. Different carburetors will work on one particular application, but if you check the oxygen sensor or the fuel trim, the mixture will be found to vary.

There are many factors that come into play, in order to correctly tune a carburetor for the optimum 14.5 to 1 mixture. A carburetor is controlled entirely by vacuum produced by the engine to draw fuel into the intake manifold. No or low vacuum results in a lean mixture. A lean mixture results when too much air and not enough fuel is introduced into the engine.

A lean mixture is the most destructive type of mixture. It is much more explosive and tends to increase the cylinder head temperature radically. When this happens, the fuel will have the tendency to ignite as soon as it enters the cylinder because of the increased temperatures. If this occurs, damage to the valves, bent connecting rods, melted pistons and so on can occur very quickly.

The mixture can by determined by the color of the spark plug electrode. A light gray color means that the mixture is optimum. A white electrode that is blistered or melted is a lean condition. A black sooty look is a mixture that is too rich. This condition results when there is too much fuel and not enough air. This will ordinarily foul the plugs, carbon the pistons and result in poor fuel economy, but very little damage will occur.

This information is necessary to diagnose and tune a carburetor for optimum performance. Once an understanding of the things that effect a carburetor's performance is acquired, it is easy to perform service on them. With a fuel-injected engine, the computer continuously adjusts the mixture for all engine demands. It does this by monitoring the air density, temperature, altitude and the condition of the engine. A carburetor can not do this since everything is fixed position. This being the case, it becomes obvious how the best possible medium is the best that can be acquired.

The components on a carburetor are:

  • The idle mixture screws (these govern the amount of fuel being added to the air at an idle), which are located in the front lower base of the carburetor or on both sides of many four barrel carburetors.
  • The mid range jets, which are usually fixed and are used in conjunction with the idle mixture for part throttle operation. They are seen just above the throttle plates.
  • The main metering jets, which are the power jets used during any acceleration or at high engine RPM. They are always located in the float bowl--either in the bowl itself or on the carburetor body below the fuel level. These are always the protruding orifice at the upper level of the inside of the throttle bore. Removing the top of the carburetor or float bowl exposes these jets, which can be changed to a million sizes (so it seems) to achieve the best power and mixture at mid to high RPM.
  • The accelerator pump is always in the float bowl. It has a protruding rod with which to either extend or lessen the stroke of the pump. When the throttle is opened on any engine, the vacuum momentarily drops to 0. When this happens, no fuel is being drawn into the engine and this is where the accelerator pump comes in. The pump picks up fuel from the float bowl and pumps a stream of fuel into the carburetor to offset the momentary lack of fuel. Without the accelerator pump, the engine would fall on its face for a second every time the engine is attempting to accelerate. While looking down the inside of the carburetor bore, push the throttle linkage open quickly and a large stream of fuel can be seen squirting into the engine.
  • The last major component is the choke. The choke plate is the large flap at the top of the carburetor. It is used to help create enough vacuum (when the engine is starting) to draw fuel into the manifold when there is very low vacuum. It is controlled by a round cover on the passenger side of the carburetor that contains an electrically heated coil spring. When the engine is cold, the spring contracts and closes the choke. When the key is turned on and the engine started, the electrical power running through the spring heats the spring and makes it expand thus pushing the choke open slightly. As soon as the engine is running fast enough to produce vacuum, a vacuum chamber on the passenger side of the carburetor called the choke pull down, is drawn back by the vacuum and pulls on a rod that opens the choke further to raise the rpm. As the engine heats up, the choke continues to open until it is all the way open.

The four-barrel carburetor, as opposed to the two-barrel carburetor simply has two extra bores and throttle plates. These throttle plates are activated by vacuum or manually through the throttle linkage. They are also adjustable through different tension springs on the vacuum operated or bending a rod on the manually operated ones. Some carburetors have a separate float bowl for the secondaries and others share one float for both. Many hi performance carburetors have a second set of mixture screws located in the side of the carburetor.

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