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Carburetor Basics

The main function of the carburetor is to atomize the liquid fuel into a gas for combustion creating the power stroke of the engine. On an alternate downward stroke of the piston, air is drawn through the venturi increasing its velocity, in turn creating a vacuum that draws fuel from a jet. The air and fuel are atomize in the venturi and delivered to the combustion chamber for the compression stroke and ignition

Model A Zenith Carburetor throttle
throttle plate

The throttle plate manages the amount of air flow that is delivered to the engine and is controlled by the "Throttle Lever" and "Accelerator" inside the cab.

Model A Zenith Carburetor choke

Pulling on the Carburetor Adjustment Knob, often called the GAV (Gas Air Valve) inside the cab opens and closes the Choke Plate. Turning the GAV enriches or leans the fuel mixture flowing through the "Cap Jet".

Model A Zenith Carburetor starting choke

When the engine is started with the choke closed a greater vacuum is formed, pulling in a larger amount of fuel.

Model A Zenith Carburetor hidden passages
  • GREEN: Adjustable fuel supply for Cap Jet
  • BLUE: Predetermined fuel supply by Compensator Jet for Cap Jet and Idle Jet
  • RED: Direct fuel supply for Main Jet (Used for High Speeds)
  • YELLOW: Ambient air to fuel bowl
Model A Zenith Carburetor compensating jet
compensating jet

The “Compensating Jet” is inside the fuel bowl and empties into the “Secondary Well” which is open to the air. The “Cap Jet” connects with the “Secondary Well”. The “Cap Jet” can only draw as much fuel as the “Compensating Jet” will allow, regardless of the amount of suction.

Model A Zenith Carburetor cap jet
cap jet

The "Cap Jet" has an additional fuel supply from the "Fuel Bowl". The flow rate of the fuel to the "Cap Jet" is controlled inside the cab with the "GAV" adjustment knob. Turning it left (towards the driver) enriches the fuel mixture for the "Cap Jet". The "Cap Jet" is used at low speeds.

Model A Zenith Carburetor main jet
main jet

The “Main Jet” is connected directly to the "Fuel Bowl". It acts like a straw; the stronger the suction the greater amount of fuel will be supplied. The "Main Jet" kicks in and helps out the "Cap Jet" at higher speeds. When cruising, the "Cap Jet" can be leaned out (turn right) to conserve on fuel.

Model A Zenith Carburetor idle
idle jet

Idling occurs when the "Throttle Plate" is partially open. A vacuum above the plate is created, drawing air through a small hole, which pulls the gas from the secondary well through the "Idle Jet".

Model A Zenith Carburetor exploded schematic diagram

Before blaming the carburetor check the following:

Model A Zenith Carburetor poor idle fuel leak por mileage Zenith Carburetor poor idle fuel leak por mileage