Husqvarna K 750 Workshop Manual page 34

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CYLINDER/PISTON
Piston damage
The cause of engine failure is often difficult to establish,
primarily when the machine's history is not known The typical
cases below can provide some guidance.
Normal wear
Typical normal wear is easiest to see on the piston sections that
face the exhaust and inlet sides. From the detailed image it is
evident that the piston has been "polished" to give a bright sur-
face, yet the machining lines after manufacture are still visible.
Oiled piston rings indicate correct lubrication. The piston rings
have full moveability in the piston ring groove.
Dust
Dust entering the engine will dramatically shorten its life. The
effect of dust can be clearly seen if the piston is studied under a
magnifying glass. Spiral lines in line with the piston's travel are
a clear sign. The machining lines after manufacture cannot be
seen. The surface is matt.
Usual cause is inadequate filter service and/or leakage. Check
the condition of the filters and gaskets. Also check the rubber
guides between the cylinder and the carburettor as well as the
connections. Methodical searching for dust, from the filter
units to the inlet by the cylinder, should give a result.
(The sooty piston top indicates that the machine works at
short intervals and has not been run completely warm.)
Scoring
Damage of this type is always the result of overheating. The
cores are usually on the exhaust side, which is the hottest. Inlet
side can show similar damage.
Check whether the machine has broken down due to an
incorrect oil mixture, or no oil at all. The oil on the overheated
piston has probably carbonized. Check instead whether the
connecting rod or the crankcase has a film of oil.
If a lack of oil can be excluded, inlet leakage should be
looked at. When the engine has air leakage on the inlet side,
this results in a lean fuel/air mixture which first and foremost
give scoring on the exhaust side. Look for combination effect
as in the example below.
Overheated piston top
Clear signs of overheating are aluminium deposits on the top
of the piston, which in extreme cases can result in melting.
Excess air in relation to the fuel volume increases the tempera -
ture in the combustion chamber. Therefore check whether the
inlet system is blocked. Check that the fuel supply has not been
obstructed due to a defective fuel hose, tank vent or the pulse
hose.
Fuel with a too low octane grade cause ignition at the wrong
point like spark plugs with the wrong thermal rating, which
both increase the temperature in the combustion chamber.
Check that the flywheel's key on the crankshaft is intact, as
the flywheel position on the crankshaft controls the ignition
time point.
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