Toro 51643 Service Manual page 23

Carburetor Operation The Diaphragm Pump
(cont'd)
2.
The inlet flapper valve is pulled open to allow fuel
to flow from the fuel line and into the fuel inlet
cavity.
3.
Fuel is drawn into the fuel pump cavity.
As the piston moves downward, another sequence
occurs which pumps the fuel from the pump cavity
into the the main diaphragm cavity. See Figure 9.
FUEL PUMP CAVITY
I
INLET NEEDLE
Figure 9
1.
The exhaust stroke sends a positive pressure pulse
to the pump diaphragm which pushes the
diaphragm toward the pump cover.
2.
The positive pressure in the fuel pump cavity seals
the inlet flapper valve to prevent fuel from being
forced back into the fuel line.
3. The outlet flapper valve is pushed open.
4.
Fuel is forced out of the pump cavity and past the
inlet needle.
NOTE: The inlet needle is spring loaded in the
closed position. The spring pressure is very slight
and is easily overcome by the force of the fuel
coming through the passage.
5.
The fuel enters the main diaphragm cavity. See
Figure 10.
NOTE: The main diaphragm serves only as a
reservoir so that fuel can be properly metered
before entering the venturi. It does not do any
pumping of fuel during normal operation.
6.
Once the positive pressure is gone, the needle
valve is reseated. This allows fuel
to be drawn from
the sealed main diaphragm cavity and into the
carburetor throat on the intake stroke.
12
MAIN DIAPHRAGM
CAVITY INLET NEEDLE
Figure 10
Carburetor Operation Fuel Metering Mixing
With the main diaphragm filled with fuel, the carburetor
meters the amount going into the engine by using the
high speed mixture screw and the needle jet. Before
entering the intake port of the engine, the fuel is mixed
with air to further atomize
it
and increase its
combustibility.
The metering and mixing operation is described below:
See Figure
11.
n
THROTTLE VALVE
SECONDARY
NEEDLE JET
HIGH SPEED
MIXTURE s c r e w
CAVITY
Figure
11
1.
On the intake stroke, vacuum in the carburetor
throat draws fuel from the main diaphragm cavity.
Note that as the intake stroke begins, the main
diaphragm cavity is filled with fuel and that the
diaphragm is bowed toward the cover.