Chapter 4: The Fuel System and Governor
44
8. Carefully pry out the metering plug using a small
screwdriver. See Figure 4.46.
9. Examine the metering plug: See Figure 4.47.
• Fuel, drawn from the central column via the long
fuel feed leg, is metered by the brass orifice in
the tip of the metering plug.
• Air, drawn from the emulsion air port, is metered
by the size of the brass orifice at the entrance to
the port.
• The fuel and air that feed the pilot and transition
ports are mixed at the metering plug.
• The metering plug creates a small venturi. The
pressure drop of the air passing through the
metering plug draws the fuel into the passage to
the pilot and transition ports, in an emulsified
mixture.
NOTE: In cut-away view, the passage by the
metering port is visible. See Figure 4.48.
NOTE: The pilot screw regulates how much of
this pre-mixed fuel/air emulsion is allowed to
enter the throat of the carburetor, to atomize
down-stream of the throttle plate. On early pro-
duction carburetors the pilot screw is service-
able, but on current production units it is set at
the factory with loctite to prevent movement from
vibration. The screw head is removed to meet
EPA and CARB requirements. See Fig-
ure 4.49.
NOTE: The transition ports are fixed. They are
drilled into the throat of the carburetor, down-
stream of the venturi. They lie behind the brass
welch plug near the pilot screw.
Figure 4.46
Figure 4.47
O-ring seals
Air passage
End view
Fuel metering orifice
Figure 4.48
Figure 4.49
Transition ports Pilot port
Pilot screw
(old style)
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