MTD Engine - Series 350/450/650
28
8.34. Examining the metering plug: See Figure 8.34.
• 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.
8.35. In cut-away view, the passage by the metering
port is visible. See Figure 8.35.
8.36. 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. See Figure 8.36.
8.37. 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.
8.38. Adjustment on installation:
• Gently turn the pilot screw in all the way, then
back it out 3/4 turn (270 degrees).
• Tighten the idle speed screw until 1/8” (3 mm) of
the screw is visible on the throttle arm side of the
housing.
• After installation, adjust the speed screw as indi-
cated in step 5.5 of the MAINTENANCE AND
ADJUSTMENT INFORMATION section of this
manual to achieve a 1,800 RPM idle, +
160
RPM.
Figure 8.34
O-ring seals
Air passage
End view
Fuel metering orifice
Figure 8.35
Figure 8.36
Transition ports Pilot port
Pilot screw
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