Compression Test - MTD P90 series Shop Manual

P90 Series Vertical Shaft Engines

Compression test

To perform a compression test:
NOTE: Compression should be in the range of 55 - 80 PSI (3.8 - 5.5 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" extension 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.
24
Compression Readings
Readings in
psi
<20 Most likely a stuck valve or
(1.4 Bar) too tight of a valve lash,
provided the starter rope
pulls with normal effort.
20 - 55 Valve seat damage or pis-
(1.4-3.8 Bar) ton ring and/or cylinder
wear.
55 - 80 Normal readings
(3.8-5.5 Bar)
>80 Excessive valve lash, a
(>5.5 Bar) partial hydraulic lock, a bad
cam or a bad automatic
compression relief.
Possible causes
Compression gauge
Figure 2.2