Service Manual

Single Stage Snowthrower Service Manual 9 - 9
GAS POWER SHOVEL
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. Refer to Figure 201.
Figure 201 0530-05
Fuel System
Diaphragm Carburetor Theory
The Toro Gas Power Shovel is equipped with a Walbro
WTA-2 Diaphragm Carburetor. The Walbro Carburetor
operates on pressure differentials or alternating
pressure – vacuum pulses.
As the engine’s recoil starter is pulled, piston
movement in the cylinder causes the carburetor fuel
pump diaphragm to fluctuate.
Diaphragm movement causes fuel to be drawn from
the gas tank through the fuel inlet valve in the fuel
pump diaphragm. The fuel inlet valve opens during a
pressure pulse to allow fuel flow to the fuel outlet valve
in the pump diaphragm which is closed during a
pressure pulse.
As a vacuum pulse is developed within the carburetor,
the fuel inlet valve closes and the outlet valve opens,
which allows fuel to flow through the filter screen to the
metering chamber.
During the vacuum pulse, the metering diaphragm is
drawn up which causes the metering lever to pivot on
the metering lever pin. This in turn, causes the inlet
needle to lift off of the seat which allows fuel to flow to
the metering chamber. As the metering chamber fills
with fuel, spring pressure on the metering lever closes
the needle valve. Refer to Figure 204 to Figure 214.
Engine vacuum pulses then draw fuel up through the
nozzle check valve and the nozzle well and forces the
fuel into the carburetor throat. Increased air velocity at
the nozzle which is created by the venturi, develops a
low pressure area and atmospheric pressure pushes
fuel into the throttle bore to deliver the fuel to the
crankcase.
CARBURETOR ADJUSTMENT
Adjusting Carburetor
The carburetor has been adjusted at the factory, but an
adjustment may be required to compensate the
differences in fuel, temperature, and altitude.
1. Mixture Adjustment Screw (Figure 202) – Close
the screw by gently rotating it clockwise until a
slight seating resistance is felt. Next, rotate the
mixture adjusting screw one turn
counterclockwise.
IMPORTANT: Do not close the screw too tight
because the screw and seat will likely be
damaged.
Figure 202 0406-019