Automotive Systems

Formerly Automotive Systems I

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High-Speed System

High-Speed System

The, high-speed system, also called the main metering system, supplies the engine air-fuel mixture at normal cruising speeds. This system begins to function when the throttle plate is opened wide enough for the venturi action. Air flow through the carburetor must be relatively high for venturi vacuum to draw fuel out of the main discharge tube. The high-speed system provides the leanest, most fuel efficient air-fuel ratio. It functions from about 20 to 55 mph or 2,000 to 3,000 rpm.

fig0422.gif (19483 bytes)The high-speed system is the simplest system. It consists of the high-speed jet, the main discharge passage, the emulsion tube, the air bleed, and the venturi.

  • The high-speed jet is a fitting with a precision hole drilled into the center. This fitting screws into a threaded hole in the fuel bowl. One jet is used for each air horn. The hole size determines how much fuel flows through the system. A number is stamped on the high-speed jet to denote the diameter of the hole. Since jet numbering systems vary, refer to the manufacturer’s manual for information on jet size.
  • The emulsion tube and air bleed add air to the fuel flowing through the main discharge tube. The premixing of air with fuel helps the fuel atomize, as it is discharged into the air horn.
  • The venturi is the hourglass shape, formed in the side of the carburetor air horn. One or two booster venturis (fig. 4-25) can be added inside the primary venturi to increase vacuum at lower engine speeds.

The basic operation of the high-speed system is as follows:

  1. When the engine speed is high enough, air flow through the carburetor forms a high vacuum in the venturi. The vacuum pulls fuel through the main metering system.
  2. The fuel flows through the main jet that meters the amount of fuel entering the system. The fuel then flows into the main discharge tube and emulsion tube.
  3. The emulsion tube causes air from the air bleed to mix with the fuel. The fuel, mixed with air, is finally pulled out the main nozzle and into the engine.
Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

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