6
46
Cylinder and piston
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 tempera-
ture. 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 pis-
ton 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.
Cause:
• Wrong type of two-stroke oil or petrol.
• Incorrect oil mixture in the petrol.
• Incorrect carburettor setting.
Piston scoring caused by heavy carbon deposits
Too heavy carbon depositing can cause damage similar to that caused by insuf
fi
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 be-
come 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.
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.
Change to a fuel with the correct oil mix-
ture.
Correct the carburettor setting
Summary of Contents for 345FR
Page 1: ...Workshop manual English 545RX 545FR 545FX 545FXT 545RX 545RXT 345FR ...
Page 2: ......
Page 16: ...2 14 Electrical system ...
Page 52: ...6 50 Cylinder and piston ...
Page 58: ...7 56 Crankshaft and crankcase ...
Page 62: ...8 60 List of tools ...
Page 65: ......
Page 66: ...2012W12 115 48 40 26 ...