Troy-Bilt 90 Automobile Parts User Manual


 
FUEL SYSTEM AND GOVERNOR
55
12. Examine the metering plug: See Figure 4.33.
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.
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 current production units, it is
set at the factory and the screw head is removed.
See Figure 4.34.
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.
13. Soak the Carburetor body in a suitable solvent until
clean.
NOTE: Ultrasonic cleaning using a suitable water/deter-
gent mixture will clean carburetors safely and
effectively.
14. Rinse it thoroughly.
15. Dry the carburetor body using compressed air.
16. Reassemble the carburetor and install it by following
steps 1-8 in reverse order.
17. Start the engine and check the idle RPM using a
tachometer.
18. Check the top no load speed of the engine.
NOTE: The top no-load speed of the engine is 3500
RPM’s
+ 100.
19. The top no-load speed is easily adjusted by tighten-
ing/loosing the speed adjustment screw. Tighten the
screw to decrease speed and loosen it to increase
speed.
See Figure 4.35.
Figure 4.33
Air passage
O-rings
End view
Fuel metering orifice
Figure 4.34
Transition ports Pilot port
Pilot screw
(before head
is removed)
Figure 4.35
Adjustment screw
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