Yamaha YBR250 2007 Service Manual page 73

CHECKING THE COMPRESSION
3-17
6. Measure:
• compression pressure
Out of specification
(d).
Compression (at sea level)
Standard 1200Kpa
(12kg/cm 2, 170.7 psi)
Minimum 1050Kpa
(10.5kg/cm
Maximum 1300Kpa
(13.0kg/cm
a. Set the starting 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.
V V V V V
WARNING
To prevent sparking, ground all spark plug
leads 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
-->
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
CHK
ADJ
Refer to steps (c) and
-->
2
, 152.3 psi)
2
, 188.5 psi)
Eliminate.
Diagnosis
Piston ring(s) wear or
damage --> Repair.
Piston, valves, cylinder
head gasket or piston
possibly defective -->
Repair.