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Air leaks often cause prob-
lems for pro saw users, yet few
understand what one is. Even
experienced operators often
confuse the symptoms of an air
leak with those of a carburetor
problem -- a misdiagnosis that
causes them to loose productive
work time and spend money
unnecessarily on replacement
parts their engine doesn't need.
The following information
explains what an air leak is,
why they occur, and how to
remedy a saw engine that has
one.
One reason crankcase air
leaks are not well understood is
because they are problems
unique to two-cycle engines.
Most other engines -- including
other small engines -- are
immune. Their crankcases are
vented to the atmosphere, so
they run at atmospheric pres-
sure. Saw engines are not, and
when air leaks into the crank-
case, it causes problems. If your
saw's engine over-revs, over-
heats, stalls on accelerations, or
fails to idle, beware. These may
be caused by an air leak.
How Air Flows Through A
Saw Engine
To gain an understanding
of what an air leak is, it helps to
know how air flows through a
two-cycle engine. (If two-cycle
theory is mumbo-jumbo to you,
skip to the heading: Where Air
Leaks Occur.)
Everyone knows air begins
its journey through a saw's
engine at the air filter. After
flowing through the filter, it
travels to the carburetor, where
it is mixed with a small amount
of fuel. After exiting the carbu-
retor, it waits for the intake port
on the cylinder to open.
As the air/fuel mixture is
waits for the port to open,
inside the engine's cylinder, the
piston is traveling up the cylin-
der. This is causing the volume
of air in the crankcase to
expand. This expansion creates
a vacuum in the crankcase.
When the piston nears the top
of its stroke, its skirt uncovers
the intake port. With the port
open, the vacuum in the crank-
case sucks the waiting air/fuel
mixture into the engine's lower
crankcase area.
When the piston reaches the
top of its stroke, it reverses
direction. On its way down, the
piston skirt (bottom of the
piston) closes the intake port,
trapping the air/fuel mixture in
the crankcase. As the piston
continues down the cylinder, it
compresses the air/fuel mixture
that flowed in just a moment
ago.
As the piston nears the
bottom of its stroke, it passes by
the transfer ports, opening
them. With these ports open,
the compressed air/fuel mix-
ture flows up into the engine's
combustion chamber.
At the bottom of the stroke,
the piston reverses direction
again. On its way back up, it
closes the transfer ports and
compresses the air/fuel mixture
in the combustion chamber.
As the piston nears the top
of the cylinder, the spark plug
ignites the compressed mixture.
The burning air/fuel mixture
creates heat and expanding
gases push down on the piston.
This is what rotates the crank
shaft and makes engine power.
If you have never tinkered
with a two-cycle engine, the last
few paragraphs may not be
meaningful. Fortunately, a full
understanding is not really
important for understanding air
leaks. (If you are a visual
learner and want to understand
two-cycle theory better, there
are several excellent animations
available on the internet.)
Instead, notice how many times
the words "pressure" and
"vacuum" were used. Even
without a thorough understand-
ing of two-cycle air flow, it is
easy to see that an air leak
would dilute both pressure and
vacuum.
Where Air Leaks Occur
Air leaks can be caused by
many things, but common cul-
prits are the engine's main bear-
ing seals, the cylinder's base
gasket, or the crankcase gasket.
Even the impulse hose, which
runs from the crankcase to the
carburetor, is often an offender.
The best way to diagnose an air
leak is with a pressure test. This
How To Troubleshoot An Air Leak On A Pro Saw
This hand air pump can produce
both vacuum and pressure. Both
are necessary for testing the integ-
rity of a saw's maincase.