104
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
a chain saw may have the fuel tank and bar oil
reservoirs built into the engine housing.
As of 2012, the plastic used to construct the
fuel tank must meet the EPA evaporative emis-
sions standards for non-permeability. Fuel per-
meation occurs when hydrocarbon molecules in
the fuel work their way through the walls of a
plastic fuel tank and enter the atmosphere.
Fuel Tank Vent
To supply fuel to the engine, the fuel tank must
be vented to allow atmospheric pressure to
enter the tank during engine operation. Other-
wise, as the fuel is pulled from the tank, a vac-
uum would be created and eventually prevent
fuel delivery to the engine. The fuel tank vent, or
tank vent, allows atmospheric pressure to enter
the fuel tank but does not allow fuel vapors to
leave the tank. The tank vent has a one-way
check valve that closes when pressure inside
the tank increases above atmospheric pressure.
During engine operation—engine running and
operator movement—the fuel in the tank is agi-
tated, causing some of the fuel to vaporize and
creating vapor pressure. Without the one-way
tank vent check valve, the pressurized vapors,
consisting of unburned hydrocarbons, would be
emitted into the atmosphere.
The fuel tank vent protrudes outside the fuel
tank and is installed directly into the wall of the
tank or connected to the tank by a fuel line. Fuel
line is used in the tank vent system because it
is compatible with gasoline. The fuel line used
must also meet the EPA emission standards for
evaporative emissions and not allow any hydro-
carbons to permeate through the fuel line and
enter the atmosphere.
Fuel Tank Lid
Also known as the tank lid or cap, the fuel tank
lid seals the fuel tank during engine opera-
tion but can be removed to resupply the tank
with fuel. The tank lid must not allow fuel to
spill or leak out of the fuel tank when the two-
stroke engine is operated in different positions.
Engine rollout refers to a two-stroke engine that
is rotated up and away from normal position to
the left or to the right. Sometimes, a two-stroke
engine is operated in a completely inverted posi-
tion, a 180° rollout. The lid should not allow any
fuel to leak from the tank.
Older two-stroke engines had tank caps that
were vented. These types of caps should not be
used as replacements on newer engines that
must meet EPA emission standards. The old
lid may cause the new tank to leak and allow
unburned hydrocarbons to be emitted to the
atmosphere.
Fuel Filter
The function of the fuel filter is to prevent fine r
particles from entering the carburetor and
obstructing fuel flow. Also known as the fuel
pickup body, the fuel filter typically consists of a
weight component, prefilter, fuel filter, and fuel
line connector. See Figure 5-36. The weight
component allows the fuel filter to sink to the
lowest part of the fuel tank regardless of engine
position. The prefilter is a porous material that
allows fuel to pass through while stopping large
contaminants in the fuel from entering the fuel
line. The filter portion stops smaller contami-
nants. Some fuel filter pickup bodies will com-
bine the function of a filter and a prefilter into
one porous material. The fuel line fits onto the
connector of the pickup body.
The micron rating of a filter is a way of indi-
cating what size particles the filter allows to
pass through it. One micron is one-thousandth
of a millimeter. The smaller the micron number,
the smaller the filter openings and the finer the
particles the filter will stop. The filter mesh also
creates resistance to fuel flow, slowing it down.
Engines with smaller displacements require less
fuel than engines with larger displacements and
can use a filter with a smaller micron rating.
Larger engines need more fuel flow and require
a larger micron rated filter.
Clamp Prefilter Weight Filter screen
Goodheart-Willcox Publisher
Figure 5-36. The fuel filter prevents contami-
nants in the fuel from entering the fuel line and
going to the carburetor.