Husqvarna 125C Workshop Manual page 42

Cylinder, piston, crankshaft, crankcase
Medium to deep scores on the exhaust side.
The piston ring is stuck in the groove.
Black discoloration under the piston ring
due to blow-by.
Exhaust side damaged by a broken piston
ring. The piston ring parts damage the top
section and cause score marks.
Piston scoring caused by heavy carbon deposits
Too heavy carbon depositing can cause damage similar to that caused by insuffi-
cient lubrication. However, the piston skirt has a darker colour caused by the hot
combustion gases that are blown past the piston.
This type of piston damage starts at the exhaust port where carbon deposits can
become loose and get trapped between the piston and the cylinder wall.
Typical for this type of piston damage is brown or black discoloration of the piston
skirt.
Cause:
• Wrong type of two-stroke oil or
petrol.
• Incorrect oil mixture in the petrol.
• Incorrect carburettor setting.
Piston damage caused by a too high engine speed.
Typical damage associated with a too high engine speed includes broken piston
rings, broken circlip on the gudgeon pin, faulty bearings or that the guide pin for the
piston ring has become loose.
Piston ring breakage
A too "lean" carburettor setting results in a too high speed and a high piston
temperature. If the piston temperature rises above the normal working temperature
the piston ring can seize in its groove, consequently it will not sit deep enough in its
groove. The edges of the piston ring can then hit the top edge of the exhaust port
and be smashed and also cause piston damage.
A too high engine speed can also cause rapid wear to the piston ring and play in
the piston ring groove primarily in front of the exhaust port. The ring is weakened by
the wear and can be caught in the port causing serious piston damage.
Action:
Change the fuel.
Change to a fuel with the correct oil
mixture.
Correct the carburettor setting
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