32
B.
Engine will have to be motored to a speed of about 1000 RPM. Hold throttle wide open and take
several compression readings. Consistant readings of 110 to 120 psi indicate good compression.
INSPECTION-DISASSEMBLY
When diassembling an engine, carefully inspect and note the physical appearance of each of the
components. Often the appearance of parts will indicate operation under other than ideal conditions. In
observing these indicators, you may be able to suggest improved service and operating techniques which
will result in prolonged engine service life. Some of the things to look for are:
1.
Excessive sludge and varnish formation.
2.
Scoring of the cylinder walls.
3.
Severe piston damage.
4.
Evidence of external oil leakage.
Sludge is a natural by-product of combustion and a small accumulation is normal. Excessive sludge
formation could indicate several things. The most common cause is perhaps too infrequent oil and oil filter
changes. It can also indicate operation with improper ignition timing or overrich carburetor adjustment or
a-poorly serviced clogged air cleaner which restricts air intake and also results in an overrich mixture.
Scoring of the Cylinder Wall
Unburnt fuel not only adds to sludge formation but can, in severe cases, cause scuffing and scoring of
the cylinder walls. As raw fuel seeps down the cylinder walls, it washes the necessary lubricating oils off
the piston and cylinder walls so that the piston rings make metal to metal contact with the walls. Scoring of
the cylinder walls can also be caused by localized hot spots resulting from blocked cooling fins or from
inadequate or contaminated lubrication.
Figure 23 Remove Flywheel With Puller
Figure 24 Using Valve Compressor
Severe Piston Damage
Major damage to pistons and rings can take various forms. The top of the piston ring may be burned
through or the top groove may be excessively worn and the ring broken or stuck in the groove. This can be
attributed to abnormal combustion. If ignition timing is overadvanced, ignition will occur while the piston
still has a long distance to travel on its compression stroke. As a result, the combined heat of compression
plus the heat of preignited fuel raises temperatures to values comparable to that of an acetylene torch. This,
of course, acts mainly on the top land and top ring of the piston and results in early failure.
Evidence of External Oil Leakage
If excessive oil leakage is evident, this may indicate improperly serviced breather systems, Normally,
an engine operates internally at pressures under atmospheric or, in other words, with a negative crankcase
