Yamaha YP400T Service Manual page 102

MEASURING THE COMPRESSION PRESSURE
7. Measure:
• compression pressure
Out of specification → Refer to steps (c)
and (d).
Compression pressure (at sea
level)
Minimum
870 kPa (8.7 kg/cm
Standard
1,000 kPa
(10.0 kg/cm
Maximum
1,120 kPa
(11.2 kg/cm
M MMM M MMM M M MMM M
a. Set the main switch to "ON".
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
engine of 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 with-
out oil
Same as without
oil
L LLL L LLL L L LLL L
3 - 22
CHK
ADJ
2
, 123.7 psi)
2
, 142.2 psi)
2
, 159.3 psi)
M MMM M M MMM M M MMM M MMM
Diagnosis
Piston wear or
damage → Repair.
Piston rings,
valves, cylinder
head gasket or pis-
ton possibly defec-
tive → Repair.
L LLL L L LLL L L LLL L LLL