Yamaha YZF-R125, Service Manual

Page: 82 / 356

ENGINE
3-9
8. Install:
• Timing mark accessing screw
(along with the O-ring )
9. Install:
• Left lower side cowling
• Right side cover
• Rider seat
Refer to “GENERAL CHASSIS” on page 4-1.
EAS20710
MEASURING THE COMPRESSION
PRESSURE
NOTE:
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 3-3.
2. Start the engine, warm it up for several min-
utes, and then turn it off.
3. Remove:
• Rider seat
• Right upper side cowling
Refer to “GENERAL CHASSIS” on page 4-1.
4. Remove:
• Fuel tank
Refer to “FUEL TANK” on page 7-1.
5. Disconnect:
• Coolant temperature sensor coupler “1”
• Spark plug cap “2”
6. Remove:
• Spark plug
CAUTION:
ECA5D71007
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.
7. Install:
• Extension “1”
• Compression gauge “2”
8. Measure:
• Compression pressure
Out of specification
→
Refer to steps (c) and
(d).
▼▼▼ ▼ ▼ ▼▼▼ ▼ ▼ ▼▼▼ ▼ ▼ ▼▼▼ ▼ ▼ ▼▼▼ ▼ ▼ ▼▼▼ ▼ ▼▼▼
a. Set the main switch to “ON”.
b. With the throttle wide open, crank the engine
until the reading on the compression gauge
stabilizes.
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.
New
2
1
Extension
90890-04082
Compression gauge
90890-03081
Engine compression tester
YU-33223
Standard compression pressure
(at sea level)
550 kPa/600 r/min (78.2 psi/600
r/min) (5.5 kgf/cm²/600 r/min)
Minimum–maximum
480–620 kPa (68.3–88.2 psi)
(4.8–6.2 kgf/cm²)
1
2
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