Service Manual

GAS POWER SHOVEL
9 - 8 Single Stage Snowthrower Service Manual
In a loop-scavenged engine with a third-port, the flow
of the fuel/air mixture can be described as follows:
(Figure 197).
Figure 197 0406-018
1. The fresh fuel/air mixture enters the crankcase
through the third-port on the side of the cylinder.
The oil in the fuel/air mixture lubricates the moving
parts within the crankcase.
2. The fuel/air mixture is transported through the fuel
transfer ports to the combustion chamber where it
is compressed, ignited, and then expelled from the
engine through the exhaust port.
Engine Operation
The piston closes all engine ports as it moves toward
the combustion chamber. The moving piston creates
high pressure in the combustion chamber and partial
vacuum in the crankcase. Refer to Figure 198.
Figure 198 0539-02
At a point slightly before top dead center (BTDC) of the
piston travel, the spark from the plug ignites the fuel/air
mixture. Also, at this time, the third-port opens
allowing the fresh fuel/air mixture to rush into the
crankcase to equalize the partial vacuum. Refer to
Figure 199.
Figure 199 0530-03
The expanding gases from the burning fuel in the
combustion chamber force the piston down the
cylinder, 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 gases begin to be expelled from the combustion
chamber. Refer to Figure 200.
Figure 200 0530-04