Small Engine Compression

Test the compression
on your small engine.

Small Engine Compression

A compression test is used
to determine the health
of a small engine.

Testing Compression

Small engines use a piston
in a cylinder where the power
is produced.

The piston travels up and down
in the cylinder through four
cycles, intake, compression,
ignition, and exhaust.

On the compression stroke,
the piston travels upward
in the closed enclosure,
compressing the fuel air

This is the compression
pressure developed that
we measure.

To measure small engine
compression we use a
compression gauge.

On a engine in good shape
the compression should be
between 90 and 100 pound
per square inch (psi).

To test for compression the
spark plug must be removed.

Before removing the spark
inspect for any dirt or foreign
material that could fall into
the spark plug hole when
it is removed.

Clean around the spark plug.

The spark ignition wire must
be removed from the spark plug.

The rubber cap may be sealed
onto the spark plug and you
need to work it lose by
twisting and pulling upward
on the insulating cap;
not the wire.

Remove the spark plug,
set it aside, and place the
ignition wire out of the way.

Place the compression gauge
onto the spark plug hole,
press firmly to seal the
connection, and give the
rope a few pulls.

Most compression gauges
have a valve to hold the
pressure until you are
ready to release it.

Remove the pressure gauge
and study the results.

Some small engine pressure
gauges screw into the spark
plug hole and you do not
need to hold it by hand while
doing the test.

A quick and dirty test if you
do not have a compression
gauge is to place your thumb
over the spark plug hole
and give the starting rope
a pull.
The cylinder pressure should
try to lift your thumb off
and release pressure.

No compression could indicate
burnt valves, leaking head gasket,
broken rings, worn valve guides
or a hole in the top of the piston.


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How to check the
compression of a small
gasoline engine.,
at findnchoose.

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