Engine Inspection; Measuring The Compression Pressure - Yamaha SR400 2014 Service Manual

EAS2RD1039

ENGINE INSPECTION

EAS20720
MEASURING THE COMPRESSION
PRESSURE
The following procedure applies to checking and
adjusting the compression pressure.
TIP
Insufficient compression pressure will result in a
loss of performance.
1. Measure:
• Valve clearance
Out of specification Adjust.
Refer to "ADJUSTING THE VALVE CLEAR-
ANCE" on page 3-5.
2. Start the engine, warm it up for several min-
utes, and then turn it off.
3. Remove:
• Spark plug cap
4. Remove:
• Spark plug
ECA20470
NOTICE
Before removing the spark plug, use com-
pressed air to blow away any dirt accumulat-
ed in the spark plug well to prevent it from
falling into the cylinder.
5. Install:
• Compression gauge "1"
Compression gauge
90890-03081
Engine compression tester
YU-33223
Extension
90890-04082
1
6. Measure:
• Compression pressure
Out of specification  Refer to steps (b) and
(c).
ENGINE INSPECTION
Standard compression pressure
(at sea level)
1050 kPa/700 r/min (10.5
kgf/cm
r/min)
Minimum–maximum
910–1180 kPa/700 r/min
(9.1–11.8 kgf/cm
129.4–167.8 psi/700 r/min)
▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼
a. With the throttle wide open, kick the kickstart-
er lever until the reading on the compression
gauge stabilizes.
EWA2RD1003
WARNING
To prevent spark generation, spark plug lead
should be grounded before cranking by the
kickstarter.
b. If the compression pressure is above the
maximum specification, check the cylinder
head, valve surfaces, and piston crown for
carbon deposits.
Carbon deposits  Eliminate.
c. If the compression pressure is below the min-
imum specification, pour a teaspoonful en-
gine 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 without
oil
Same as without oil
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7. Install:
• Spark plug
Spark plug
25 Nm (2.5 m·kgf, 18 ft·lbf)
8. Connect:
• Spark plug cap
5-1
2
/700 r/min, 149.3 psi/700
2
/700 r/min,
Diagnosis
Piston ring(s) wear or
damage  Repair.
Piston, valves, cylin-
der head gasket or
piston possibly defec-
tive  Repair.