MTD P90 series Shop Manual page 45

Welch plug
Shot plug in
feed bore
Fuel port to
central column
Figure 4.11
Spacer
Metering
plug
Figure 4.12
O-rings
Air passage
Figure 4.13
Throttle stop
screw
Fuel feed leg
on central
column for pilot
and transition
End view
Fuel metering orifice
F
7. The throttle stop screw has a large pliable lip around
the head of the screw. That lip secures a metering
plug for the pilot and transition ports. Remove the
screw to reach the plug. See Figure 4.11.
8. Carefully pry out the metering plug and spacer using
a small screwdriver. See Figure 4.12.
NOTE: Do not remove the throttle shaft. The screws that
hold the throttle plate to the throttle shaft are
peened. When they are removed, they will expand
the holes in the throttle shaft, ruining it.
NOTE: Do not try to remove the choke shaft. The plastic
end of the shaft is molded over a grove to hold it in
place. Prying on it will break the shaft.
9. Examine the metering plug: See Figure 4.13.
• 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 meter-
ing plug draws the fuel into the passage to the pilot
and transition ports, in an emulsified mixture.
S G
UEL YSTEM AND OVERNOR
39