Husqvarna 131RB Workshop Manual page 55

Item Piston damage because of a too high engine speed
Too high engine speed can cause damage to the piston rings, the snap ring on the wrist pin and the
bearings. Too high engine speed can also loosen the guide pin for the piston ring.
A too lean carburetor setting results in too high speed and a high piston temperature. If the piston tem-
perature increases too much, the piston ring is blocked and does not go down sufficiently in its groove.
The edges of the piston ring can then hit the top edge of the exhaust port and break and cause piston
damage. A too high engine speed also causes damage to the piston ring and play in the piston ring
groove, usually in front of the exhaust port. The wear can cause the piston ring to catch in the exhaust
port and cause damage to the piston.
4 The figure shows the exhaust side damaged by a broken piston ring. The piston ring parts damage the
top section and cause score marks.
5 A too high engine speed causes the ends of the piston ring to hit the guide pin again an again. This can
push the pin through the top of the piston and cause damage to the cylinder.
6 A too high engine speed causes vibration to the snap rings of the wrist pin. The snap ring pulls out of its
groove and the tension of the snap ring decreases. The snap ring becomes loose and causes damage to
the piston.
7 Failure of the crankshaft bearing or on the connecting rod bearing is usually caused by a too high engine
speed. This results in the overload to the bearing or that the bearing becomes too hot. Overload to the
bearing can prevent rotation of the bearing rollers and break the roller or ball retainer. The broken mate-
rial causes a blockage between the piston and cylinder wall and causes damage to the piston skirt. Un-
wanted material can also move through the supply channels of the cylinder. This causes damage to the
combustion chamber of the cylinder and to the top and sides of the piston.
627 - 002 - 23.04.2018 Repair instructions - 55