Chapter 2: Basic Troubleshooting
18
Compression test
To perform a compression test:
NOTE: Compression should be in the range of
45-95 PSI (3.1-6.5 Bar).
• Disconnect the high-tension lead from the spark
plug and ground it well away from the spark plug
hole.
• Remove the spark plug using a 13/16” or 21mm
wrench. A flexible coupling or “wobbly” exten-
sion may help.
• Pull the starter rope several times to purge any
fuel or oil from the combustion chamber.
NOTE: Air compresses readily, liquid does not.
Liquid in the combustion chamber will result in
an artificially high compression reading.
1. Install a compression gauge in the spark plug
hole.
2. Confirm that the gauge is “zeroed”, then pull the
starter rope repeatedly, until the needle on the
gauge stops rising. See Figure 2.2.
3. Interpreting compression readings.
Figure 2.2
Compression gauge
Reading ~
90 PSI
Table 2:
Readings in
psi
Possible causes
<20
(1.38 Bar)
Most likely a stuck valve or
too tight of a valve lash,
provided the starter rope
pulls with normal effort.
20-35
(1.38-2.4 Bar)
Valve seat damage or pis-
ton ring and/or cylinder
wear.
35-95
(2.4-6.5 Bar)
Normal readings
>95
(>6.5 Bar)
Excessive valve lash, a
partial hydraulic lock, a bad
cam or a bad automatic
compression relief.
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