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.
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.
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.
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.
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.
operation of the high-speed system is as follows:
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.
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.
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.
by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy