nope.. a petcock doesn't regulate flow.
Most carbs have a float bowl and a float (operates much like a like a toilet bowl, where water enters full blast)
fuel travels through the fuel hose and enters a hole in the float bowl (called a float needle valve). The float rises and closes the valve with a "float needle".. so fuel shuts off when it reaches the proper level.
float level partly determines how much fuel is mixed with the air streaming through the carburetor..