Yamaha YFM250RX 2008 Service Manual page 79

MEASURING THE COMPRESSION PRESSURE
6. Measure:
• Compression pressure
Out of specification → Refer to steps (c)
and (d).
Compression pressure (standard)
1,100 kPa/720 r/min
(11.2 kgf/cm
Compression pressure
(maximum)
1,300 kPa/720 r/min
(13.3 kgf/cm
Compression pressure (minimum)
900 kPa/720 r/min
(9.2 kgf/cm
a. Set the main switch to "ON".
b. With the throttle wide open, crank the
engine until the reading on the compres-
sion 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 again.
Refer to the following table.
Compression pressure
(with oil applied into the cylinder)
Reading
Higher than without
oil
Same as without oil
3-18
CHK
ADJ
2
, 160 psi/720 r/min)
2
, 189 psi /720 r/min)
2
, 131 psi /720 r/min)
Diagnosis
Piston ring(s) wear or
damage → Repair.
Piston, valves, cylin-
der head gasket or
piston possibly
defective → Repair.