Lesson 1/learning Event 2
With the throttle closed, the engine creates a high vacuum below the throttle valve because the valve itself
acts as a restriction. The low-speed circuit has an idle discharge hole below the throttle valve. The higher
air pressure on the fuel in the carburetor bowl pushes the fuel to the vacuum at the idle discharging hole.
A small air-bleed hole is located at the top of the low-speed circuit passage. It lets air bleed into the circuit
and mix with the gasoline. This helps atomize the fuel before it leaves the idle discharge port.
An adjusting needle valve is placed at the idle discharge hole. Turning this screw changes the richness of
the idle mixture. Turning the screw inward closes the hole and allows less fuel to flow. With less fuel
flowing, a leaner mixture is created. When the screw is turned outward, more fuel flows and the mixture is
Opening the throttle a little above idle speed allows more air to pass the throttle valve. However, there is
still not enough air passing through the venturi to get fuel to flow through the main discharge nozzle. More
air is passing the throttle valve, so more fuel must be added to keep the mixture correct. To let more fuel
in, a low-speed discharge port is added just above the idle discharge hole. As the throttle is opened slightly,
its edge moves above the low-speed discharge hole. More fuel enters the airstream because it can now pass
through two discharge holes. (In some carburetors, a long discharge slot is used instead of holes.)
The high-speed circuit includes the venturi, the main nozzle, and the high-speed jet. Usually, more than one
venturi is used in a carburetor, with one placed inside the other. The main nozzle is centered in the smallest
venturi. Fuel passes from the carburetor bowl to the main nozzle and flows through the high-speed jet.
The hole in the jet is drilled to an exact size to meter the fuel flowing in the high-speed circuit.
In operation, the high- and low-speed circuits overlap. As the throttle is moved, each circuit continues
working until after the next one begins. If there is no overlap between the circuits, the engine will run
rough or even try to stop at certain throttle positions. This condition is called a flat spot in the carburetor.