Service Manual

ENGINE TOP END 5-5
Cylinder Head
Compression Measurement
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Before measuring compression, do the following:
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Be sure the battery is fully charged.
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Thoroughly warm up the engine so that engine oil between the piston
and cylinder wall will help seal compression as it does during normal
running.
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Stop the engine.
•
Disconnect the spark plug caps of each cylinder and remove the spark
plugs.
•
Attach the compression gauge assembly firmly into one plug hole.
Special Tools - Compression Gauge: 57001–221 [A]
Compression Gauge Adapter: 57001–1159 [B]
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Ground the spark plugs to the engine.
To avoid fire, do not ground the spark plugs in proximity to the
plug holes. Keep the plugs as far away as possible from the
plug holes.
•
With the throttle fully open, turn engine over with the recoil starter
several times until the compression gauge stops rising; the compres-
sion is the highest reading obtainable. (Recoil Starter Model)
Cylinder Compression (MIN) 390 kPa (57 psi)
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Using the starter motor, turn the engine over with the throttle fully open
until the compression gauge stops rising; (Electric Starter Model).
Cylinder Compression (MIN) 390 kPa (57 psi)
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Cylinder compression reaches more than 1000 kpa (145 psi) in a
case when cranking speed is over 700 rpm.
•
Repeat the measurement to the other cylinder.
If the compression is higher than the specified value, the piston rings,
cylinder and valves are probably in good condition.
If the compression is too high, check the following.
1. Carbon build-up on the piston crown and cylinder head - clean off
any carbon on the piston crown and cylinder head.
2. Cylinder head gasket - use only the proper gasket. The use of a
gasket of incorrect thickness will change the compression.
3. Valve guides and piston rings - rapid carbon accumulation in the
combustion chamber may be caused by worn valve guides and/
or worn piston oil rings. This may be indicated by white exhaust
smoke.
If cylinder compression is lower than the (MIN), check the following:
1. Gas leakage around the cylinder head - replace the damaged
gasket and check and check the cylinder head warp.
2. Condition of the valve seating.
3. Valve clearance.
4. Piston/cylinder wear, piston seizure.
5. Piston ring, piston ring groove.