O.S. Engines 25XZ - Manual (Page 15)

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Dirt and dust may lodge in marked places.
5.Finally, when cleaning the exterior of the
engine, use methanol or a household
cleaning agent. Do not use gasoline,
kerosene, or any petroleum based chemical
which can damage silicone fuel tubing.
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CHECKING THE ENGINE
If the engine will not develop normal performance
after long time running due to wearing of parts.
It is suggested to replace necessary parts when
the following symptoms are detected.
Engine sound changes and easily overheats.
Power has dropped extremely.
Idle is unstable and/or engine tends to stop
at idle.
In most cases, ball bearings, cylinder & piston
assembly, connecting rod and/or crankcase have
become worn. Check the parts carefully and
replace them if necessary.