Yamaha 25BMH, Service Manual

Page: 142 / 446

POWER UNIT
3-19
E
CHK
ADJ
POWER UNIT
MEASURING THE COMPRESSION
PRESSURE
NOTE:
The engine should be warmed up before
measuring the compression pressure. Cor-
rect measurement cannot be obtained
when the engine is cold.
Measure:
• Compression pressure
Below minimum compression pres-
sure
→
Check or replace piston, pis-
ton rings, cylinder head, and
cylinder head gasket.
Measuring steps
(1) Remove the spark plug .
CAUTION:
Before removing the spark plug, clean the
spark plug well arec to prevent any dirt
from falling into the cylinder that is being
checked.
(2) Install the compression gauge into
the spark plug hole.
(3) Fully open the throttle.
(4) Fully open the choke. (Make the choke
valve ineffective.)
(5) Crank the engine by means of recoil
starter until the compression gauge
gives stable reading.
(6) Remove the compression gauge .
(7) Install the spark plug .
1
2
Compression pressure
(reference value)
25B: 730 kPa
(7.3 kgf/cm
2
, 105.9 psi)
30H: 740 kPa
(7.4 kgf/cm
2
, 107.3 psi)
Minimum compression pressure
25B: 580 kPa
(5.8 kgf/cm
2
, 84.1 psi)
30H: 590 kPa
(5.9 kgf/cm
2
, 85.6 psi)
Compression gauge ......................
90890-03160
T
R
.
.
Spark plug
25 N
•
m (2.5 kgf
•
m, 18 ft
•
lb)
1
2
2
2
1