Engine Inspection; Measuring The Compression Pressure - Yamaha UMAX Service Manual

ENGINE INSPECTION

MEASURING THE COMPRESSION
PRESSURE
TIP
Insufficient compression pressure will result in a
loss of performance.
1. Measure:
• Valve clearance
Out of specification → Adjust.
Refer to "CHECKING THE ENGINE COOL-
ING INLET DUCT (YUM1A/YUM2A)" on
page 3-4.
2. Start the engine, warm it up for several min-
utes, and then turn it off.
3. Remove:
• Seat frames (driver and passenger)
Refer to "GENERAL CHASSIS (1)" on page
4-1.
• Drive belt
Refer to "REPLACING THE DRIVE BELT
(YUM1A/YUM2A)" on page 3-8.
4. Disconnect:
• Spark plug cap
5. Remove:
• Spark plug
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:
• Extension "1"
• Compression gauge "2"
Extension
90890-04082
Compression gauge
90890-03081
Engine compression tester
YU-33223
2
1
ENGINE INSPECTION
7. Measure:
• Compression pressure
Out of specification → Refer to steps (b) and
(c).
Compression pressure
700–900 kPa (7.0–9.0 kgf/cm²,
102–131 psi)
a. Crank the engine by fully pressing the ac-
celerator pedal until the reading on the
compression gauge stabilizes.
WARNING
To prevent sparking, ground the spark plug
lead before cranking the engine.
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
minimum specification, pour a teaspoonful
of engine 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
8. Install:
• Spark plug
Spark plug
20 N·m (2.0 kgf·m, 15 lb·ft)
TIP
Before installing the spark plug, clean the spark
plug and gasket surface.
9. Connect:
• Spark plug cap
10.Install:
• Drive belt
Refer to "REPLACING THE DRIVE BELT
(YUM1A/YUM2A)" on page 3-8.
• Seat frames (driver and passenger)
Refer to "GENERAL CHASSIS (1)" on page
4-1.
5-1
Diagnosis
Piston ring(s) wear or
damage → Repair.
Piston, valves, cylinder
head gasket, or piston
ring(s) possibly defec-
tive → Repair.