BASIC TROUBLESHOOTING
17
Compression test
To perform a compression test:
NOTE: Compression should be in the range of
40-60 PSI (2.81-4.1 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 ~
40 PSI
Table 2:
Readings in psi Possible causes
<20
(1.38 Bar)
Most likely a stuck valve or
too tight of a valve lash, pro-
vided the starter rope pulls
with normal effort.
20-35
(1.38-2.4 Bar)
Valve seat damage or piston
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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