Carburetor Basics

The carburetor was used as the first method of mixing air and fuel to the correct ratio for combustion. They were used in the original automobile and just about every car after that until the 1980s when fuel injection started to take over. They were completely phased out by the end of the 1990s, even earlier in North America. Carbs worked well as an “all-in-one” fuel management system. They did not rely on any electronics, they were fully mechanical and did a fairly decent job. They were phased out and replaced by fuel injection because of emission standards and rising fuel prices. The reason carburetors couldn’t compete with fuel injection is because they were clumsy compared to modern fuel injection systems. A carburetors only inputs are airflow into the engine and throttle position to determine how much fuel to add, but a fuel injection system can have many different inputs to determine the engines needs and supply the (almost) exact amount of fuel needed.As a cylinder pulls air into the engine on intake stroke, a low pressure area or vacuum is created in the intake manifold. A carburetor uses this low pressure area to pull fuel into the airstream. There are 3 sources of vacuum that a carburetor uses under different conditions to meter fuel into the engine: manifold, ported and venturi. Manifold vacuum is the strongest, it is vacuum below the throttle plate when it is closed. An engine tries to pull in as much air as it can, but the throttle plate is blocking most of it. This results in an extreme low pressure area in the intake manifold at idle, but not much at part to full throttle. Ported vacuum acts as a switched vacuum source, as the throttle plate opens it allows vacuum to be applied to other devices. Proted vacuum is 0 at idle, strong at cruiding speed and weak at full throttle. Venturi is the most difficult to understand, first you must understand that as airflow passes through a restriction, its speed is increased and its pressure is reduced. This results in a vacuum only when enough air is pulled through a venturi or restriction (high RPM).

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Carb Circuits

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Float Circuit

Carburetor float circuitThe float circuits job is to maintain a set amount of fuel in the bowl at all times. The fuel pump supplies fuel at a low pressure, (about 5-15psi) if the carb is using less fuel than the pump is delivering the bowl will fill up. When the bowl gets too full the float will block fuel flow into the bowl until some of the fuel is used up.

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Idle Circuit

Carburetor idle circuitThe idle circuit is able to pull fuel under the throttle plate when it is closed because of the low pressure/high vacuum area created in the intake manifold. The amount of fuel that enters the air stream in idle controls idle speed and idle quality. This is adjusted by the idle mixture screw.

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Low Speed Circuit

Carburetor low speed circuitWhen the operator presses the accelerator, the throttle plate starts to open. It uncovers the off-idle port, exposing it to ported vacuum. This allows fuel to be pulled through the off-idle port.

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Main Metering Circuit

Carburetor main metering circuitWhen the operator demands a moderate amount of throttle, the throttle plate opens further and there is enough airflow to create vacuum in the veturi and pull fuel through the main jet and discharge tube.

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Power Circuit

Carburetor power circuitWhen the operator demands wide open throttle (WOT), the metering rod moves completely out of the way of the main jet to allow extra fuel to be pulled through the discharge tube.

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Acceleration Circuit

Carburetor acceleration circuitWhen the operator opens the throttle quickly the engine could run lean for a moment before fuel is pulled through the main jet. To counteract this as well as provide instant throttle response, the acceleration pump will push a small amount of extra fuel through the pump nozzle when the throttle is opened quickly for sudden acceleration.

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Choke Circuit

Carburetor choke circuitWhen the engine is cold the choke plate is closed to expose the discharge tube to manifold vacuum and pull fuel through it. If the throttle opened to the point that the engine needs more air than what can get passed the choke plate, the choke plate is opened until the throttle plate is returned to a slightly open position.

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