3.4
Section 3
Troubleshooting
Compression Test
A compression test can be performed using a
compression tester. Follow the manufacturers
instructions for performing the test.
Cylinder Leakdown Test
A cylinder leakdown test can be a valuable alternative
to a compression test. By pressurizing the combustion
chamber from an external air source you can
determine if the valves or rings are leaking, and how
badly.
Cylinder Leakdown Tester (see Section 2) is a
relatively simple, inexpensive leakdown tester for
small engines. The tester includes a quick disconnect
for a aching the adapter hose, and a holding tool.
Leakdown Test Instructions
1. Run engine for 3-5 minutes to warm it up.
2. Remove spark plug(s) and air fi lter from engine.
3. Rotate the cranksha until the piston (of
cylinder being tested) is at top dead center of
the compression stroke. Hold the engine in this
position while testing. The holding tool supplied
with the tester can be used if the PTO end of
the cranksha is accessible. Lock the holding
tool onto the cranksha . Install a 3/8 in. breaker
bar into the hole/slot of the holding tool, so it
is perpendicular to both the holding tool and
cranksha PTO.
If the fl ywheel end is more accessible, use a
breaker bar and socket on the fl ywheel nut/
screw to hold it in position. An assistant may be
needed to hold the breaker bar during testing. If
the engine is mounted in a piece of equipment,
it may be possible to hold it by clamping or
wedging a driven component. Just be certain
that the engine cannot rotate off of TDC in either
direction.
4. Install the adapter into the spark plug hole, but
do not a ach it to the tester at this time.
5. Connect an air source of at least 50 psi to the
tester.
6. Turn the regulator knob in the increase
(clockwise) direction until the gauge needle is in
the yellow set area at the low end of the scale.
7. Connect the tester quick-disconnect to the
adapter hose while fi rmly holding the engine
at TDC. Note the gauge reading and listen for
escaping air at the thro le body intake, exhaust
outlet, and crankcase breather.
8. Check your test results against the following
table:
Leakdown Test Results
Air escaping from crankcase breather ...................................Defective rings or worn cylinder.
Air escaping from exhaust system .........................................Defective exhaust valve/improper seating.
Air escaping from thro le body..............................................Defective intake valve/improper seating.
Gauge reading in Low (green) zone .......................................Piston rings and cylinder in good condition.
Gauge reading in Moderate (yellow) zone ...........................Engine is still usable, but there is some wear present.
Customer should start planning for overhaul or
replacement.
Gauge reading in High (red) zone .........................................Rings and/or cylinder have considerable
wear. Engine
should be reconditioned or replaced.
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