Yamaha XT660R S Service Manual page 104

MEASURING THE COMPRESSION PRESSURE
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6. Measure:
• compression pressure
Out of specification → Refer to steps (c)
and (d).
Compression pressure (at sea
level)
Minimum
600 kPa (6.0 kg/cm
Standard
650 kPa (6.5 kg/cm
Maximum
700 kPa (7.0 kg/cm
M MMM M MMM M M MMM
a. Set the main switch to "ON" and the engine
stop switch to "
b. With the throttle wide open, crank the
engine until the reading on the compression
gauge stabilizes.
WARNING
_
To prevent sparking, ground the spark plug
lead before cranking the engine.
c. If the compression pressure is above the
maximum specification, check the cylinder
head, valve surfaces, and piston crown for
carbon deposits.
Carbon deposits → Eliminate.
d. If the compression pressure is below the
minimum specification, pour a teaspoonful
of engine oil into the spark plug bore and
measure it again.
Refer to the following table.
Compression pressure
(with oil applied into the cylinder)
Reading
Higher than with-
out oil
Same as without
oil
L LLL L LLL L L LLL
3 - 22
CHK
ADJ
2
, 85.3 psi)
2
, 92.4 psi)
2
, 99.6 psi)
M M MMM M M MMM M
".
Diagnosis
Piston ring(s) worn
or damaged →
Repair.
Piston, valves, cyl-
inder head gasket
or piston possibly
defective → Repair.
L L LLL L L LLL L
M MMM M MMM
L LLL L LLL