Carburetor Theory And Operation - Toro GTS 120 Service Manual

the exhaust ports. The third port design engine
has the carburetor mounted on the side of the
cylinder. The passage from the carburetor into
the crankcase is called the third port. All ports
within the engine are opened and closed by the
piston skirt as the piston moves up and down
within the cylinder.
Operation
The piston closes all engine ports as it moves
toward the combustion chamber (Figure 1-1). The
moving piston creates a high pressure in the com-
bustion chamber and a partial vacuum in the
crankcase.
Intake Ports Closed
Co
Figure 1-1
At a point slightly before top dead center (BTDC) of
the piston travel, from the plug ignitesthe
fuel air mixture (Figure 1-2). Also, at this time, the
third port opens allowing the fresh fuel/air mixture
to rush into the crankcase to equalize the partial
vacuum.
Intake Ports Closed
\
Figure 1-2
When the fuel is ignited by the spark plug, the
expanding gases from the burning fuel in the com-
bustion chamber force the piston down thecylinder,
closing the third-port and increasing the pressure
in the crankcase. At a point approaching the bottom
of the stroke, the exhaust port opens and the burnt
1-3
gases begin to be expelled from the combustion
chamber (Figure 1-3).
__-
- - l _ _ _ _ _ m _
Intake Ports Closed
Exhaust Port
I
Third Port Closed
~
Figure 1-3
At a point of piston travel slightly before bottom
dead center (BBDC) the fuel transfer ports, on the
sides of the cylinder walls, are uncovered and
the compressed fuel/air mixture in the crankcase
-is allowed to enter the combustion chamber
where they help expel the burnt gases and
charge the chamber for the following piston
stroke (Figure 1-4).
Intake Ports
Open
Exhaust
Change
__-__
Figure 1-4
CARBURETOR THEORY AND OPERATION
Theory
The carburetor receives fuel from the tank and
mixes it with air in the right proportions to
provide a highly combustible mixture to the
engine.
As the piston moves up on the compression
stroke a partial vacuum is created within the
engine crankcase, causing the greater atmos-
pheric pressure to force air to flow through the
carburetor into the cylinder. The velocity of the
air increases as it flows through the carburetor
venturi and the air pressure is reduced at this
point to less than atmospheric pressure. The
differences of pressure in the venturi of the