provides the leanest, most fuel efficient air-fuel ratio. It
functions from about 20 to 55 mph or 2,000 to 3,000
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
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 manufacturers
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.
Figure 4-25.Booster venturi
The basic operation of the high-speed system is as
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
Figure 4-26.Mechanically operated metering rod.