204
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
following are questions the repair technician
could discuss with the customer:
“How long ago did the customer purchase
the fuel?” With the presence of alcohol in the ”
fuel, the gasoline can become stale in as little as
30 days. If the customer purchased the fuel and
stored it for longer than 30 days, the fuel may
have decomposed to the point it would not run
in a two-stroke engine. Also, when the fuel was
purchased, it could have been in the gas station’s
tank in the ground for over 30 days already.
“Where did the customer purchase the
fuel?” The frequency with which a gas station ”
refills their storage tanks determines how fresh
the fuel is in their tanks. A gas station that sells
a low volume of fuel may go more than 30 days
before refilling their storage tanks. This allows
time for the gasoline in the tanks to decompose
and become stale, especially with the addition
of ethanol. A gas station next to a busy highway
probably sells a large volume of fuel and replen-
ishes their storage tanks frequently with fresh
fuel. If fuel is purchased from a local low-vol-
ume gas station, it is possible that the fuel could
already be stale enough to prevent a two-stroke
engine from starting and running.
“In what type of container does the customer
store the fuel?” A proper consumer fuel-storage ”
container must be sealed and nonpermeable.
Older fuel tanks may be vented. This allows the
lighter gasoline molecules to evaporate, leav-
ing behind a less-combustible fuel. Venting also
allows oxygen to come in contact with the gaso-
line, assisting in the process of decomposition.
Pure gasoline may remain in a vented storage
container for 90 days before showing signs of
decomposition. If the gasoline contains ethanol,
oxygen in the ethanol accelerates the decompo-
sition process, causing the fuel to become sig-
nificantly stale in as little as 30 days.
On hot summer days, a vented gasoline con-
tainer heats up during the day and cools during
the night. During cooling, humid air may be
pulled into the tank. Ethanol in the fuel is
hygroscopic and absorbs this moisture and
adds this moisture to the gasoline.
Even with sealed fuel storage containers,
lightweight hydrocarbon molecules still evap-
orate out of the gasoline. If the consumer
opens the container, these molecules escape
to the atmosphere. The gasoline left behind is
degraded. It is important to shake the container
before opening it to dissolve these lightweight
molecules back into the gasoline.
“During which season was the fuel pur-
chased?” From season to season, different ”
additives are dissolved into the gasoline to
increase or decrease the volatility of the gaso-
line, depending on the typical local conditions.
In the hot days of summer, inhibitors are added
to the gasoline to reduce the volatility of the gas-
oline. Imagine a consumer purchases this gaso-
line and stores it for later use. If the consumer
uses this fuel in a two-stroke engine in late
fall or winter, the engine may have a very hard
time starting because of the lack of volatility of
the gasoline. The additive package is seasonal.
Some refer to these packages as “summer gas”
or “winter gas.” In some areas, there may be as
many as 12 different seasonal additive packages
for the gasoline.
Exhaust System Evaluation
The next step in engine evaluation is to evaluate
the exhaust system. Remove the spark arrester
mesh. The mesh should be clean, with only min-
imal brown ash. Carbon deposits indicate an
engine that has been running rich, with incor-
rect premix oil, or with an incorrect oil-to-fuel
ratio. See Figure 9-47. Abrasives in the engine
can also create a buildup on the spark arrester
mesh. A fouled mesh can allow an engine to
Carbon buildup
Goodheart-Willcox Publisher
Figure 9-47. An obstructed spark arrester mesh
may allow an engine to start and run, but not at
full throttle. The mesh can be cleaned, but it is
more important to fix the situation that caused
the buildup.