Evinrude 4906B 4HP 1969, Руководство пользователя

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A fuel chamber vent hole is located near
the top of the carburetor body to permit
atmospheric pressure to act against the fuel
in each chamber. This pressure assures an
adequate fuel supply to the various operat-
ing systems of the engine.
Air
IFuel
Mixture
A suction effect is created each time
the piston moves upward in the cylinder.
This suction draws air through the throat of
the carburetor. A restriction in the throat,
called a venturi, controls air velocity and
has the effect of reducing air pressure at
this point.
The difference in air pressures at the
throat and in the fuel chamber, causes the
fuel to be pushed out metering jets extend-
ing down into the fuel chamber. When the
fuel leaves the jets, it mixes with the air
passing through the venturi. This fuel/air
mixture should then be in the proper propor-
tion for burning in the cylinder /s for maxi-
mum engine performance.
In order to obtain the proper air/fuel
mixture for all engine speeds, high- and low-
speed needle valves are installed. On late-
model engines, the high-speed needle valve
was replaced with a high-speed orifice.
There is no adjustment with the orifice
type. These needle valves are used to
compensate for changing atmospheric condi-
tions. Probably over 50% of the engines
covered in this manual have an adjustable
high- and low-speed needle valve.
FLOAT
Fuel [tow controlled
by
the float in
a
typical carbu-
retor. The inlet valve is opened and closed by move-
ment
of
the float in the carburetor bowl.
CARBURETION 4-3
ATMOSPHERIC AIR PRESSURE
Air flow principle
of a
modern carburetor.
Engine operation at sea level compared
with performance at high altitudes is quite
noticeable. A throttle valve controls the
volume of air/fuel mixture drawn into the
engine. A cold engine requires a richer fuel
mixture to start and during the brief period
it is warming to normal operating tempera-
ture. A choke valve is placed ahead of the
metering jets and venturi to provide the
extra amount of air required for start and
while the engine is cold.
Choke valve location
in
the carburetor venturi. The
choke valve on most Johnson/Evinrude carburetors is
located in front
of
the venturi.