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 TM 5-2805-261-13
The compressed air-fuel mixture ... now highly combustible ... is
throat or barrel. A narrowing in the carburetor throat called the
ignited by the spark plug when a timed electric current jumps
venturi causes the air passing through to speed up. Because
the gap between its electrodes.
View C illustrates the
the speed of the air moving through the venturi is increased, the
downward or power stroke of the piston under the force of the
air pressure in the venturi is reduced.
explosion. As the piston ... in its downward travel ... uncovers
the exhaust and intake ports, the intake and exhaust stage
shown in View A again occurs. With an outboard motor engine
running at full throttle this cycle may be repeated more than
4500 times per minute.
Figure 1-5. View C
B. FUEL SYSTEM. The function of the fuel system is to
store, supply, filter and mix gasoline (and lubricant)
with air for combustion in the cylinders. The fuel
system is composed of the fuel tank, the fuel pump,
the carburetor, the leaf plate assembly, and the lines,
hoses, and connectors that carry fuel from component
to component.
Figure 1-6. Carburetor (1 of 2)
1.
The carburetor is a single-barrel, float feed type
with a manual choke, fixed high speed jet, and
The difference in air pressure over the high speed nozzle and
an adjustable slow speed needle valve. The
atmospheric pressure in the float chamber, causes the fuel to
throttle valve is synchronized with the ignition
be pushed up the high speed nozzle and into the air stream in
system by a throttle control cam located on the
the throat of the carburetor.
armature plate and the cam follower located on
the carburetor body. The cam follower is linked
The throttle and choke valves work together to control the
to the throttle valve shaft.
amount of fuel-air mixture fed to the crankcase and the ratio of
fuel to air. The choke valve is used to control the ratio of air to
The carburetor mixes fuel and air in proportions determined by
fuel by controlling the air intake. The throttle valve controls the
engine speed requirements. Fuel is held in a small, bowl-like
amount of fuel fed to the crankcase.
chamber. A cork float operates the inlet valve which allows fuel
to be replaced as it is consumed.
The fixed high speed jet insures an even flow of fuel up the high
speed nozzle. The fuel must pass through the high speed jet
Piston movement creates alternating conditions of pressure and
before it rises up the high speed nozzle.
partial vacuum in the crankcase. Atmospheric pressure trying
to fill the partial vacuum creates a flow of air through the
The high speed jet is fixed and needs no adjustment in its
carburetor
orifice. The slow speed needle valve is adjustable.
1-6
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