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TESTING FOR OVERHAUL
HOW TO DETERMINE ENGINE OVERHAUL PERIOD
Cause of Low Compression
Generally, the time at which an engine should be overhauled
is detennined
by
various conditions such as lowered engine
power output, decreased compression pressure, and increased
fuel and oil consumption. The lowered engine power output
is not necessarily due to trouble with the engine itself, but is
sometimes caused by improper oil, clogged filters or a faulty
carburetor.
The decrease in compression pressure is caused by many fac-
tors.
It
is. therefore, necessary to detennine a cause or causes
on the basis of data produced by periodic inspection and
maintenance.
Oil
analysis on a seasonal basis is a good
means of monitoring engine internal wear. When caused by
worn cylinders or piston rings, the following symptoms will
occur:
1 Low engine power output
2 Increased fuel consumption
3 Increased oil consumption
4 Hard engine starting
5 Noisy engine operation
These symptoms often appear together. Symptoms 2 and 4
can result also from improper fuel regulation or a faulty car-
buretor. They are caused also by defective electrical devices
such as the battery, starter or spark plugs. Therefore it is
desirable to judge the optimum engine overhaul time by the
lowered compression pressure caused by worn cylinders and
pistons plus increased oil consumption. Satisfactory combus-
tion is obtained only under sufficient compression pressure.
If an engine lacks compression pressure, incomplete combus-
tion of fuel will take place even if other parts of the engine
are operating properly. To detennine the period of engine
overhaul, it is important to measure the engine compression
pressure regularly. At the same time, the engine speed at
which the measurement of compression pressure is made
should be checked because the compression pressure varies
with engine rpm. The engine rpm can be measured at the
front end of the crankshaft.
NOTE:
To test engine compression see the ENGINE
ADJUSTMENT section of this manual.
OVERHAUL CONDITIONS
co~pressioln pressure tendsdt0alincrease ahli \:-':::"e in a new
engme untl piston f1ngs an v ve seats a
v-
~
b
b k
'
Thereafter, it decreases gradually with the
p""~geen
rfo en
10.
f th
arts
~
ress 0 wear
o
esep
.
When decrease of compression pressure reac::: ---e tl
.
.
I · .
h '
b
h
led
-0.1 S
lelepalr
mut, t e engIne must e over au
.
The engine requires overhaul when oil
consL.~
t'
. h' I
.
.
....·.IIpIOI1IS
19l
blowby eVident, and compression values
are
~t
..
'
b 1
-
mlmmum or
eow.
Engines & Gener.tors
3
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