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The Place for Model-A Ford Carburetors

Understanding the theory on how this carburetor functions is very helpful in diagnosing problems.

How the carburetor basically works

  • A carburetor is that part of a gasoline engine which provides the mixture of gasoline and air that the engine burns. The carburetor must mix the gasoline with about 15 times its weight in air for the engine to run smoothly at all speeds. A driver controls the engine speed by increasing or reduction the flow of the fuel mixture.
  • Carburetors are called updraft or downdraft according to their position. If the carburetor is below the intake manifold's input, it is updraft. If it is above, it is a downdraft. The Zenith carburetor, which was used on the Model A, is an updraft.
  • The float chamber of the carburetor stores a small amount of gasoline, which is gravity-fed from the Model A fuel tank. When the carburetor bowl chamber is filled to the proper level, a float resting on top of the gasoline closes a valve restricting flow from the tank fuel line. As the engine consumes gasoline, the float drops. This opens the valve and lets more gasoline flow into the bowl chamber.
  • Air and gasoline are mixed in the Venturi, which sits in the carburetor throat area. The Venturi is a tube, which narrows to a small size and then widens out again, which increases the speed of the air rushing through the carburetor, and lowers its pressure. The higher air pressure in the float chamber then forces gasoline through the jets into the Venturi. The air picks up the gasoline and turns it into a vapor. Vacuum from the engines intake manifold draws the air and gasoline vapor into the engine.
  • The throttle plate valve controls the engine speed by letting more or less of the air and gasoline vapor to enter the intake manifold. The driver presses the accelerator pedal to open the throttle valve and lets up on the accelerator to close it.
  • The choke plate valve looks similar to the throttle valve, but it controls the amount of air entering the carburetor. When it partly closes the carburetor input opening, more gasoline and less air flow into the engine. Choking the carburetor makes it easier for the spark plugs to ignite the air-gasoline mixture when the engine is cold.
  • The Zenith carburetor, which Ford Motor Company selected for the Model A, addressed the significant design problem of obtaining a constant air/fuel mixture over a wide speed range. In a simple design carburetor, the flow of gasoline from the main jet increases under suction faster than the flow of air. This is due to inertia, friction, and the viscosity of the gasoline flowing through the small jets. A simple jet carburetor cannot maintain the needed air/fuel ratio under varying suction levels and becomes too rich at high suction levels. If the carburetor is adjusted to make it work well at high speeds, it will be too lean at low speeds.
  • The Zenith, Model A carburetor is more complex than the earlier, simpler carburetors commonly in use. In addition to a main jet design, which results in an air/fuel mixture that becomes richer with increased engine speed, Zenith added some other functions. The Cap Jet, Secondary Well, and Compensator Jet were added to provide a way to reduce the amount of gasoline provided to the engine as the suction (speed) increases, which is opposite of the Main Jet. These parts work by filling the Secondary Well with gasoline from the main bowl through the Compensator Jet when the engine is turned off or operating at low speeds. The Secondary well fills with gasoline and when the engine speed increases, all of the fuel in the Secondary Well is sucked into the carburetor throat through the Cap Jet. The Secondary Well provides an extra shot of fuel instantly when the throttle is opened rapidly. This flow of fuel at low speeds stops an otherwise lean mixture that would occur just off idle.
  • The third part of the Zenith design is the idling circuit, which consists of the Idle Jet and the Air Adjustment needle screw. They provide the right mixture of air and fuel when the throttle valve is essentially closed.
  • A fourth part of the Zenith design is the Gas Adjusting Valve (GAV), which provides a manual fuel adjustment for extra fuel from the main float bowl through the Secondary Well to the Cap Jet when needed. The carburetor was designed to be slightly lean to allow for high altitude driving and the GAV provided additional fuel. This extra fuel is needed at sea level, and even more so when the engine is cold.
  • For a schematic representation of how the carburetor works, see Flow Diagram.

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