To successfully mix fuel and air for burning in an engine's combustion chambers, we have to get fuel into a holding chamber, atomize it, mix it with the air, and meter (a controlled supply) the mix into the engine. We don't want the engine to rev with reckless abandon. We want the ability to control the engine's power output. We do this with throttle bores and plates that control the air/fuel mixtures flow into the combustion chambers.
FloatsWhen fuel enters the carburetor's fuel bowl (or bowls), we want to control the amount. In normal operation, we want a smooth, steady flow of fuel into the bowl. At wide-open throttle, we want a surplus of fuel in the bowl. The float is an air pocket wrapped in either brass or synthetic foam and permeated with air. It is also a lever that bears against the needle valve. The needle valve is a fuel-flow traffic cop that prevents or allows fuel flow into the carburetor. It's typically a rubber-tipped, brass valve piston that slides inside a brass cylinder bore screwed into the carburetor body. Adjust the float level by either bending the tab that bears against the needle or moving the needle-valve seat back and forth against the float tap. Float level is generally set by the carburetor manufacturer.
Float function differs depending on the kind of driving. When drag racing, fuel sloshes toward the back of the fuel bowl, sometimes away from where it needs to be. A carburetor fuel bowl and float that won't stop the flow of fuel is needed when we dump the clutch and haul mail. We also don't want a float and needle valve that are going to allow too much fuel into the bowl and stall the engine. This is why some carburetors are specific to drag racing.
With road racing, the problem is fuel moving to one side of the bowl. As with drag racing, the fuel needs to stay at the jets in order to keep a steady supply of fuel for the boosters. Shelby Mustangs and Cobras, for example, had road-racing Holley carburetors with center-pivot, float-needle valves designed to stay where they belong during hard cornering, braking, and acceleration.