Husqvarna 129C Workshop Manual page 46

Repair Instructions
Analysis and actions
Experience tells us that piston or cylinder failure due to manufacturing errors are extremely rare.
The reason is usually due to other factors, which is evident from the following.
Note the reasons for the breakdown, repair the damage and take the actions required to prevent the same
thing happening again.
Insufficient lubrication
Small to medium size scores in front of the
exhaust port
In extreme cases heat development can be so
great that material from the piston smears along
the piston skirt and even in the cylinder bore.
Generally the piston ring is undamaged and moves
freely in the ring groove
There can also be scores on the inlet side of the
piston.
Possible causes:
• Incorrect carburettor setting. Recommended
max. speed exceeded.
• Incorrect oil mixture in the fuel.
• Too low octane fuel.
Heavy carbon deposits
Too heavy carbon depositing can cause damage
similar to that caused by insufficient 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.
Possible causes:
• Wrong type of two-stroke oil or petrol.
• Incorrect oil mixture in the petrol.
• Incorrect carburettor setting.
Small to medium size scores
primarily in the middle of the
exhaust port.
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Medium to deep scores along entire piston
skirt
The piston ring starts to stick or is completely stuck
in its groove, preventing it from sealing against the
cylinder wall and causing intensive temperature
increases in the piston.
Seizure scores along the entire piston skirt on the
inlet and exhaust sides.
Possible causes:
• Incorrect oil mixture in the fuel.
• Too low octane fuel.
• Air leaks; cracked fuel hose, leaking inlet
gaskets, cracked distance piece or inlet manifold.
• Air leakage in engine body; leaking crankshaft
seals, leaking cylinder and crankcase gaskets.
• Poor maintenance; dirty cooling fins on the
cylinder, blocked air intake on the starter,
blocked spark arrestor mesh in the muffler.
For the best results we recommend
Husqvarna two-stroke oil or ready-mixed
fuel that is specially developed for air-
cooled two-stroke engines.
Mixing ratio: 50:1 (2%).
Medium to deep scores along the
entire piston skirt on the exhaust
side.
Heavy carbon deposits caused 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.