The purpose of the carburetor is to provide and meter the mixture of fuel vapor and air in relative to the load and speed of the engine. Because of locomotive temperature, speed, and load, ideal carburetion is very hard to obtain.

The carburetor provisions a small amount of a very rich fuel combination when the engine is cold and running at idle. With the throttle plate congested and air from the air cleaner incomplete by the closed choke plate, engine suction is augmented at the idle-circuit nozzle. This void draws a thick spray of petrol through the nozzle from the full float bowl, whose fuel line is closed by the float-supported spine valve. More fuel is provided when the gas handle is depressed for acceleration.

The pedal linkage opens the strangle plate and the choke plate to send air speeding up through the barrel. The linkage also depresses the accelerator pump, providing added petrol through the accelerator-circuit nozzle.

As air passes through the thin center of the barrel, called the "venturi", it produces suction that draws squirt from the cruising-circuit nozzle. The float-bowl level drops and causes the float to lean and the needle valve to open the fuel line. To cause a fluid to flow, there must be a high force area (which in this case is atmospheric pressure) and a low force area. Low pressure is less than atmospheric pressure. The average person refers to a low pressure area as a vacuum. Since the atmospheric pressure is already present, a low force area can be formed by air or liquid flowing through a venturi. The downward movement of the piston also creates a low force area, so air and gasoline are drawn through the carburetor and into the locomotive by suction created as the piston moves down, creating a partial vacuum in the cylinder. Differences between low force within the cylinder and atmospheric pressure outside of the carburetor causes air and fuel to flow into the tube from the carburetor