Evinrude 4906B 4HP 1969 Owner's Manual page 94

A fuel chamber vent hole is located near
the top of the carburetor
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
the piston moves upward in the cylinder.
This suction draws air through the throat of
the carburetor. A restriction
called a venturi, controls
has the effect of reducing air pressure
this point.
The difference
throat and in the fuel chamber, causes the
fuel to be pushed out metering jets extend-
ing down into the fuel chamber.
fuel leaves the jets, it mixes with the air
passing through the venturi.
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.
model engines, the high-speed needle valve
was replaced with
There is no adjustment
type. These needle
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
by
Fuel [tow controlled
retor. The inlet valve is opened and closed
of
ment the float in the carburetor
body to permit
is created each time
in the throat,
air velocity
in air pressures at the
When the
This fuel/air
On late-
a high-speed orifice.
with the orifice
valves are used to
a
the float in typical carbu-
by move-
bowl.
and
at
ATMOSPHERIC
Air flow principle
Engine operation at sea level compared
with performance
noticeable.
volume of air/fuel
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
extra amount of air required for start and
while the engine is cold.
Choke valve location
choke valve on most Johnson/Evinrude
located in front
CARBURETION
AIR PRESSURE
of a
modern carburetor.
at high altitudes
A throttle valve controls
mixture drawn into the
to provide the
in
the carburetor
carburetors
of
the venturi.
4-3
is quite
the
venturi. The
is