3.4
Section 3
Troubleshooting
Compression Test
These engines are equipped with an automatic
compression release (ACR) mechanism. Because of the
ACR mechanism, it is difficult to obtain an accurate
compression reading. As an alternate, use the
leakdown test described below.
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.
The tester is a relatively simple, inexpensive leakdown
tester for small engines. The tester includes a quick
disconnect for attaching 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 and air filter from engine.
3. Rotate crankshaft until piston is at top dead
center of compression stroke. You will need to
hold the engine in this position while testing. The
holding tool supplied with the tester can be used
if the PTO end of the crankshaft is accessible.
Slide the holding tool onto the crankshaft. See
TT-364-A. Install a 3/8" breaker bar into the
square hole of the holding tool, so it is
perpendicular to both the holding tool and
crankshaft PTO.
If the flywheel end is more accessible, you can
use a breaker bar and socket on the flywheel nut/
screw to hold it in position. You may need an
assistant to hold the breaker bar during testing. If
the engine is mounted in a piece of equipment,
you may be able 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 attach it to the tester at this time.
5. Connect an adequate 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 tester quick-disconnect to the adapter
hose while firmly holding the engine at TDC.
Note the gauge reading and listen for escaping air
at the carburetor intake, exhaust outlet, and
crankcase breather.
8. Check your test results against the table below:
Leakdown Test Results
Air escaping from crankcase breather .............................................. Defective rings or worn cylinder walls.
Air escaping from exhaust system ..................................................... Defective exhaust valve.
Air escaping from carburetor .............................................................. Defective intake valve.
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.
