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 4-7.7A. CARBURETOR - MAINTENANCE INSTRUCTIONS.
a.
Fluctuations 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 opening 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 throttle 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 controlled 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
4-362
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