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 TM 55-1905-220-14-8
4-7.7A.
CARBURETOR - MAINTENANCE INSTRUCTIONS.
a. Fluctations in crank-case pressure are introduced into the pump chamber (1,2) through impulse port (3).
Negative pressures move the diaphragm (4) to the pump chamber (1) side drawing fuel from the tank through
inlet check valve (5) into pump chamber (2). Outlet check valve (6) remains closed. Positive crank-case
pressures move the diaphragm (4) to pump chamber (2) side, opening outlet check valve (6), closing inlet check
valve (5), and transferring the fuel from chamber (2) side to starting well (7). The rate of fuel flow from well (7) to
adjusting chamber (8) is controlled by engine cylinder pressures open- ing or closing the inlet needle (9) by
means of the main diaphragm (10) and control lever (11). Negative pressure (suction stroke) causes the main
diaphragm (10) to act against the set load on the tension spring (12) and open the inlet needle (9) by control lever
(11) allowing fuel to enter adjusting chamber (8).
b. When the throttle valve (13) is opened for idling the fuel flow from chamber (8) is controlled by the idling
screw (14). Air taken from by-pass hole (15) is mixed with the fuel to produce a vapor. This vapor is metered to
the engine cylinder through pilot outlet (16). When the trottle valve (13) is moved more than 20% open, increased
air flow through the venturi begins to jet fuel through the main nozzle (17). As the throttle valve opens further the
fuel supply is control- led by main fuel mixture screw (18). The ball check in main nozzle-is to prevent air entering
chamber (8) during low speed operation.
c. With the choke valve (19) fully closed and the throttle valve (13) half open the increase in pressure drop
at the main nozzle (17) increases the flow of fuel from chamber (8) to the air flow to the engine cylinder. This
creates a rich mixture vapor for starting purposes.
Carburetor Legend
1.
Pulse chamber
2.
Pump chamber
3.
Impulse port
4.
Pump diaphragm
5.
Inlet check valve
6.
Outlet check valve
7.
Starting well
8.
Adjusting chamber
9.
Inlet needle
10.
Main diaphragm
11.
Control lever
12.
Tension spring
13.
Throttle valve
14.
Low fuel mixture screw
15.
By-pass
16.
Pilot outlet
17.
Main nozzle
18.
Main fuel mixture screw
19.
Choke valve
4-364
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