Lesson 1/Learning Event 2
When the engine is operating at cruising speeds, an air-fuel mixture of about 15:1 is being furnished for
good economy and reasonable power. A power system in the high-speed circuit can enrich this mixture if
necessary (to climb a hill or to operate at high speed).
Two types of power systems commonly used are the power jet and the metering rod.
In the power-jet system, the power-jet valve is controlled by a vacuum piston. When the throttle is closed,
manifold vacuum is high. The vacuum piston is moved up against spring pressure, and the power-jet valve is
closed. Opening the throttle to a point where more fuel is required lowers the manifold vacuum. Lower
vacuum allows the piston to be pulled down by the spring pressure. This opens the power-jet valve, letting
fuel flow through the power jet. This fuel bypasses the high-speed jet and increases the amount of fuel
delivered into the airstream by the main discharge nozzle.
Metering rods are another way of enriching the fuel-air ratio to get more power. The metering rod is usually
operated by linkage connected to the throttle valve. The rod has several steps or tapers on its lower end that
pass through the high-speed jet. As the throttle is opened, the rod is lifted, so smaller steps are in the jet.
This allows more fuel to pass through the jet, enriching the mixture.
Assume that a vehicle is moving down the road slowly. The throttle valve is practically closed, allowing only
a small amount of air to pass through the carburetor venturi. The driver suddenly decides to pass a car
ahead and slams the accelerator pedal to the floor. The throttle valve opens instantly. Air is very light in
weight, so it increases its speed through the carburetor very quickly. Fuel is much heavier, and it will move
slowly. Before the fuel and air flows become balanced, a very lean mixture is delivered to the engine. This
causes the engine to backfire or hesitate. To prevent backfire or hesitation, the carburetor is equipped with
an accelerating pump circuit. This circuit discharges a small amount of fuel into the airstream for a very
short period of time when the throttle is suddenly opened.
One type of accelerating pump circuit consists of a pump cylinder, a pump piston that is operated by linkage
connected to the throttle shaft, a fuel intake check valve in the bottom of the pump cylinder or well, a pump
discharge check valve, and a pump discharge nozzle to spray the fuel into the airstream in the carburetor.
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