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Carburetor - Carburetor work - Carburetor theory - Carburetor basics - Carburetor Catalytic - Carburetor fuel supply - Carburetor parts - Carburetor supersession - Carburetor variable venturi - Carburetor adjustment - Carburetor barrels

How does a carburetor work?

The goal of a carburetor is to mix just the right amount of gasoline with air so that the engine runs properly. 

If there is not enough fuel mixed with the air, the engine "runs lean" and either will not run or potentially damages the engine. If there is too much fuel mixed with the air, the engine "runs rich" and either will not run (it floods), runs very smoky, runs poorly (bogs down, stalls easily), or at the very least wastes fuel. The carb is in charge of getting the mixture just right.

On new cars, fuel injection is becoming nearly universal because it provides better fuel efficiency and lower emissions. But nearly all older cars, and all small equipment like lawn mowers and chain saws, use carbs because they are simple and inexpensive. The carburetor on a chain saw is a good example because it is so straightforward.

Carburetors are either:

• Fixed-venturi, in which the varying air velocity in the venturi alters the fuel flow; this architecture is employed in most downdraft carburetors found on American and some Japanese cars
• Variable-venturi, in which the fuel jet opening is varied by the slide (which simultaneously alters air flow). In "constant depression" carburetors, this is done by a vacuum operated piston connected to a tapered needle which slides inside the fuel jet.

Under all engine operating conditions, the carburetor must:

• Measure the airflow of the engine
• Deliver the correct amount of fuel to keep the fuel/air mixture in the proper range (adjusting for factors such as temperature)
• Mix the two finely and evenly

This job would be simple if air and gasoline (petrol) were ideal fluids; in practice, however, their deviations from ideal behavior due to viscosity, fluid drag, inertia, etc. require a great deal of complexity to compensate at exceptionally high or low engine speeds. A carburetor must provide the proper fuel/air mixture across a wide range of ambient temperatures, atmospheric pressures, engine speeds and loads, and centrifugal forces:

• Cold start
• Hot start
• Idling or slow-running
• Acceleration
• High speed / high power at full throttle
• Cruising at part throttle (light load)

In addition, modern carburetors are required to do this while maintaining low rates of exhaust emissions.

To function correctly under all these conditions, most carburetors contain a complex set of mechanisms to support several different operating modes, called circuits.

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Carburetor - Carburetor work - Carburetor theory - Carburetor basics - Carburetor Catalytic - Carburetor fuel supply - Carburetor parts - Carburetor supersession - Carburetor variable venturi - Carburetor adjustment - Carburetor barrels