50 - English
Repair instructions
Faults and causes
Cutting marks on the piston (Q)
1. Wrong carburettor setting. Too high fast idle
speed.
2. Too low octane number of petrol.
3. Too low or incorrect oil in the fuel.
carbon build-up(R)
1. Wrong carburettor setting. Too low fast idle
speed.
2. Too much or incorrect oil in the fuel.
Piston ring breakage
1. Too high engine speed.
2. Piston ring worn out.
3. Oversized piston ring groove.
Cleaning and checking
Clean all parts, scrape off all
gasket residue and carbon from the following
places:
• The piston crown
• Top of the cylinder bore (inside)
• The cylinder exhaust port
• The base of the cylinder and/or crankcase
(see fi gure 77)
Fig 78
Fig 79
Check the following:
• That the cylinder’s surface coating is not worn.
Especially the upper part of the cylinder.
• That the cylinder does not have any chafe or
cutting marks.
• That the piston is free from cutting marks.
Minor scratches can be polished off using fi ne
emery paper.
• That the piston ring is not welded to its track.
• Remove the piston ring from the piston to
measure the wear. Use the base of the piston to
push the piston ring down into the cylinder. The
opening in the ring must not exceed 1 mm. Put
the piston ring back into the piston slot and make
sure it is the right way around (see fi gure 78).
• That the gudgeon pin bearing is intact.
• That the inlet bellows is intact.
• That air hoses and impulse hose are intact.
Fig 77
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