WM, Workshop Manual, K650, K700, K950, K1250, 2001-06, Power ...

5 Cylinder and piston
Piston seizures resulting from severe carbon deposits
Excessive carbon deposits can result in damage similar to that caused by insufficient
lubrication. The piston skirt, however, is darker in colour as a result of the hot
combustion gases which are pressed past the piston ring.
This type of piston damage begins at the exhaust port where carbon deposits can
loosen and stick between the piston and cylinder wall.
Medium to deep scratches on the exhaust side.
The piston ring has stuck in its groove. Black
discolouring under the piston ring resulting from
so-called "blow through".
Typical for this type of piston damage is the brown and black discolouring of the piston
skirt.
Reasons:
● Incorrect type of two-stroke oil and/or
petrol.
● Incorrect oil mixture in the petrol.
● Incorrect carburettor setting.
Measures:
Change the fuel.
Change to fuel with correct oil mixture.
Correct the carburettor setting.
Inlet side. The piston ring has stuck in its groove.
Black discolouring under the piston ring resulting
from so-called "blow through".
Piston damage resulting from excessively high engine revs.
Typical damage resulting from excessively high engine revs include fracture of the
piston ring, broken circlip for the piston bolt, defective bearings or the loosening of the
guide pin for the piston ring.
Piston ring fracture
A too ”lean” carburettor setting results in both higher revs and higher piston temperatures.
If the piston temperature rises over the normal working temperature the piston ring
can seize in its groove, which in turn can imply that it fails to go sufficiently deep in the
groove. The edge of the piston ring may therefore hit the head edge of the exhaust port
and become broken, also resulting in damage to the piston.
Excessive engine revs can also result in rapid wearing of the piston ring and play in the
piston ring groove, primarily opposite the exhaust port. The ring is weakened by the
wear and can stick in the port, resulting in serious damage to the piston.
The exhaust side damaged by a broken piston
ring. The piston ring parts damage the head part
of the piston and result in scratch marks.
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