Husqvarna 545RXT AT Workshop Manual page 63

Cylinder and piston
14.5.2 Piston scoring caused by 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.
Cause:
Wrong type of two-stroke oil or petrol.
Incorrect oil mixture in the petrol.
Incorrect air/fuel mixture.
14.5.3 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.
14.5.4 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.
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.
Action:
Change the fuel.
Change to a fuel with the correct oil mixture.
Run Engine adjustment according to 9.9
Exhaust side damaged by a broken piston ring. The
piston ring parts damage the top section and cause
score marks.
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