CARBURETOR
Identification
The E series engines use two versions of the primer
start carburetor. One with an air filter box on the
mower engines (DuraForce) and one without an air
filter box on the snowthrower engines (R tek).
Differences include a plug in the snowthrower
carburetor vent passage and some minor differences in
the throttle shafts, jetting and throttle valve bleed hole.
Both carburetors have two circuits and use both a
choke and primer for starting. (See Figure 13.)
Figure 14
3297-012
C. Air Pressure
A. Venturi
B. Center Passage Area
The fuel/oil mix is then picked up by, and mixed with,
the moving air, the amount of fuel determined by
throttle opening and jet size. This fuel/oil/air mixture
enters the crankcase through the reed valves
(DuraForce), or ports (R tek). (See Figure 15 and
Figure 16.) When the piston moves away from the
spark plug, the cylinder intake ports are opened and
the now pressurized crankcase causes the mixture to
move into the combustion chamber.
Reed Valve
Figure 13
0720-012
Theory of Operation
As the crankshaft rotates, the piston moves back and
forth in the cylinder, alternately creating a pressure or a
partial vacuum in the crankcase. (See Figure 16.)
As the piston moves toward the spark plug, the reed
valve opens (see Figure 15) or the inlet ports are
uncovered (see Figure 16). This causes air to rush
through the carburetor throat to equalize the pressure
in the crankcase. The carburetor throat is formed into
a venturi (large at each end but with a smaller center
passage). When air rushes through the venturi, air
pressure at the center of the passage is lowered.
Figure 15
A. Reed Valve Closed C. Second Stroke
B. Exhaust Gases
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Piston Ported
By inserting a tube from the carburetor float bowl into
the venturi (see Figure 14), the lower air pressure at
the venturi causes the normal air pressure in the bowl
to push fuel up the tube.
Figure 16
A. Intake Ports Closed C. Third Port Open
B. Ignition D. Partial Vacuum
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