Tech Tip
Why So Small?
In order to figure out the why of our test, you first have to understand how a carburetor works. Here's the short answer as to why a carburetor provides a certain amount of fuel to a given engine. The fuel supply is housed in the float bowls and is introduced into the intake manifold at idle speed by transfer slots and idle discharge ports in the carburetor's baseplate. The pressure differential between the vacuum created by the running engine below the fuel supply and atmospheric pressure above the fuel supply is what sends the fuel into the intake, and the amount of fuel is restricted by the air bleeds and idle mixture screws.
Hit The GAS!1. Once you step on the gas pedal and the throttle linkage cracks open the throttle blades in the baseplate, the accelerator pump squirters at the top of the venturi feed fuel quickly into the intake manifold, which increases the engine rpm. As the engine rpm increases, the vacuum inside the venturi increases, which draws more fuel from the boosters that are placed directly in the center of the venturi. The boosters are connected to the main well passages that lead to the main jet in the fuel bowls. | 
2. The fuel from the boosters is emulsified or mixed with air via a set of air passages that are called air bleeds. The air bleeds are basically controlled air leaks at the top of the venturi. The air bleeds can have a profound effect on the air/fuel ratio of the engine because of the amount of air they mix with the incoming fuel | 
3. The venturi and booster design is what primarily influences how much fuel is drawn from the main jets inside into the engine. The larger the venturi diameter,the slower the speed of the air that's running through it into the engine. The slower air speed alters the pressure differential between the vacuum beneath the booster and the pressure above it. This is why slapping a large-cfm carburetor onto a small-displacement engine doesn't work. The engine doesn't create enough air speed through the venturi to draw the proper amount of fuel relative to the incoming volume of air, and so the engine runs lean. |
4. In order to speed up the air and increase the vacuum that draws the fuel out of the boosters, carb tuners will use several different styles and diameters of booster. According to BG Fuel Systems, here's why:
Boosters are usually produced as zinc diecastings and are available in three configurations: down leg, annular, and straight leg. The straight-leg style is rarely used. To get boosters to function effi- ciently, they need to be placed precisely in the middle of the carburetor's main venturi. This is because the bores of main venturi have a narrow waist, which causes the airflow to increase in speed. As the air speed increases, a pressure drop occurs (a depression) at the narrow waist. The boosters, which also have a narrow waist to increase the depression further, draw fuel from the carburetor's fuel bowls through the metering circuits and out into the air stream.
The style of booster chosen is usually determined by air speed: The bigger themain venturi area, the slower the air speed. To increase the air speed in a main venturi that is operating with a down-leg booster, a carburetor manufacturer or tuner might switch to an annular style. Because of its larger size, the annular booster creates a bigger restriction in the main venturi and therefore increases air speed. | | |