The main or high-speed system comes into action above fast idle, as airflow through the venturi increases. A main metering jet in the fuel in the bowl meters fuel passing into the discharge nozzle.
How much fuel leaves the nozzle depends on the pressure difference created by the air flow through the venturi. As the throttle opens, and air flow increases, and speeds up, more and more fuel is drawn from the discharge nozzle. However, the mass of air doesn’t increase in proportion with the speed, and as a result, high speeds can produce a mixture that is too rich. To correct this, more air can be added. This is called compensation by air correction.
As the throttle opens and engine speed increases, the level in the jet well falls, exposing air bleed holes in the discharge tube. Air can now mix with the fuel and stop the mixture becoming too rich. As the throttle opens further, the fuel level falls too, exposing more air holes. More air bleeds in, to maintain the correct mixture.
The size of the main jet is selected to provide the best mixture for economy, under cruising conditions. When the throttle is open wide for maximum power, a richer mixture is required. The extra fuel is provided by a power jet, with a vacuum piston and rod opening it as it is needed.
At low speeds, intake manifold vacuum is transferred through a passage to the vacuum piston. This holds the piston up, and keeps the power valve closed.
With the throttle valve fully open for full engine power, the vacuum in the intake manifold falls. A spring pushes down the vacuum piston and rod, to open the power valve. Fuel flows through a bypass jet to enter the fuel well, and add to the fuel from the main jet. This provides the extra fuel needed to enrich the mixture for full power.
