Engine Inspection; Measuring The Compression Pressure - Yamaha MBK MW125 2014 Service Manual

EAS20041

ENGINE INSPECTION

EAS30249
MEASURING THE COMPRESSION
PRESSURE
TIP
Insufficient compression pressure will result in a
loss of performance.
1. Remove:
• Center cover
Refer to "GENERAL CHASSIS (1)" on page
4-1.
2. Measure:
• Valve clearance
Out of specification → Adjust.
Refer to "ADJUSTING THE VALVE CLEAR-
ANCE" on page 3-4.
3. Start the engine, warm it up for several min-
utes, and then turn it off.
4. Disconnect:
• Spark plug cap
5. 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.
6. Install:
• Compression gauge "1"
• Compression gauge extensions S "2"
Compression gauge
90890-03081
Engine compression tester
YU-33223
Compression gauge extension S
90890-03195
2
7. Measure:
• Compression pressure
Out of specification → Refer to steps (c) and
(d).
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a. Turn the main switch to "ON".
b. With the throttle wide open, crank the engine
until the reading on the compression gauge
stabilizes.
EWA12940
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 → Eliminate.
d. If the compression pressure is below the min-
imum specification, pour a teaspoonful of en-
gine 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
1
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8. Install:
• Spark plug
9. Connect:
• Spark plug cap
5-1
ENGINE INSPECTION
Standard compression pressure
(at sea level)
1600 kPa/725 r/min (16.0
kgf/cm²/725 r/min, 227.6 psi/725
r/min)
Minimum–maximum
1392–1792 kPa/725 r/min (13.9–
17.9 kgf/cm²/725 r/min, 198.0–
254.9 psi/725 r/min)
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Diagnosis
Piston ring(s) wear or
damage → Repair.
Piston, valves, cylinder
head gasket or piston
ring(s) possibly defec-
tive → Repair.
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Spark plug
13 Nm (1.3 m·kgf, 9.4 ft·lbf)
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