17
CARBURETOR
Identification
The E series en
ines use two versions of the primer
start carburetor. One with an air filter box on the
mower en
ines (DuraForce) and one without an air
filter box on the snowthrower en
ines (R tek).
Differences include a plu
in the snowthrower
carburetor vent passa
e and some minor differences in
the throttle shafts, jettin
and throttle valve bleed hole.
Both carburetors have two circuits and use both a
choke and primer for startin
. (See Fi
ure 13.)
Fi
ure 13
0720-012
Theory of Operation
As the crankshaft rotates, the piston moves back and
forth in the cylinder, alternately creatin
a pressure or a
partial vacuum in the crankcase. (See Fi
ure 16.)
As the piston moves toward the spark plu
, the reed
valve opens (see Fi
ure 15) or the inlet ports are
uncovered (see Fi
ure 16). This causes air to rush
throu
h the carburetor throat to equalize the pressure
in the crankcase. The carburetor throat is formed into
a venturi (lar
e at each end but with a smaller center
passa
e). When air rushes throu
h the venturi, air
pressure at the center of the passa
e is lowered.
By insertin
a tube from the carburetor float bowl into
the venturi (see Fi
ure 14), the lower air pressure at
the venturi causes the normal air pressure in the bowl
to push fuel up the tube.
Fi
ure 14
3297-012
The fuel/oil mix is then picked up by, and mixed with,
the movin
air, the amount of fuel determined by
throttle openin
and jet size. This fuel/oil/air mixture
enters the crankcase throu
h the reed valves
(DuraForce), or ports (R tek). (See Fi
ure 15 and
Fi
ure 16.) When the piston moves away from the
spark plu
, the cylinder intake ports are opened and
the now pressurized crankcase causes the mixture to
move into the combustion chamber.
Reed Valve
Fi
ure 15
3297-021-3
Piston Ported
Fi
ure 16
3297-002
A. Venturi
B. Center Passa
e Area
C. Air Pressure
A. Reed Valve Closed
B. Exhaust Gases
C. Second Stroke
A. Intake Ports Closed
B. I
nition
C. Third Port Open
D. Partial Vacuum
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