Automotive Systems

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Full-Power System

The full-power system provides a means of enriching the fuel mixture for high-speed, high-power conditions. This system operates, for example, when the driver presses the fuel pedal to pass another vehicle or to climb a steep hill. The full-power system is an addition to the high-speed system. Either a metering rod or a power valve (jet) can be used to provide variable, high-speed air-fuel ratio.

A metering rod is a stepped rod that moves in and out of the main jet to alter fuel flow. When the rod is down inside the jet, flow is restricted and a leaner fuel mixture results. When the rod is pulled out of the jet, flow is increased and a richer fuel mixture results for more power output. The metering rod is either mechanical-linkage or engine-vacuum operated.

• The mechanical linkage metering rod (fig. 4-26) is linked to the throttle lever. Whenever the throttle is opened wide, the linkage lifts the metering rod out of the jet. When the throttle is closed, the linkage lowers the metering rod into the jet.
• The vacuum operated metering rod (fig.4-27) that is controlled by engine vacuum is connected to a diaphragm. At steady speeds, power demands are low and engine vacuum is high, and the piston pushes the metering rod into the jet against spring pressure, restricting the flow to the discharge tube. When the load increases, vacuum decreases, causing the piston spring to lift the metering rod out of the jet, progressively increasing the flow of fuel to the discharge tube.

A vacuum power jet valve (fig. 4-28), also known as an economizer, performs the same function as a metering rod; it provides a variable high-speed fuel mixture. A power jet valve consists of a fuel valve, a vacuum diaphragm, and a spring. The spring holds the power valve in the normally OPEN position. A vacuum passage runs to the power valve diaphragm. When the power valve is open, it serves as an extra jet that feeds fuel into the high-speed system.

When the engine is cruising at normal highway speeds, engine intake manifold vacuum is high. This vacuum acts on the power valve diaphragm and pulls the fuel valve closed. No additional fuel is added to the metering system under normal conditions; however, when the throttle plate is swung open for passing or climbing a hill, engine vacuum drops. The spring in the power valve can push the fuel valve open. Fuel flows through the power valve and into the main metering system, adding more fuel for more engine power.

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