TM 5-4240-501-14&P
THEORIES OF OPERATION
Ignition
Spark plug cables are molded into the coil so that
moisture cannot short out the spark as could happen on
older coils that had an open connection between coil and
spark plug cable.
We would like to point out that at one time some
mechanics would try to judge the condition of the
magneto system by the brightness and the noise or
"snap" of the spark. This is not a good criterion as you
can quickly demonstrate by using a resistor type spark
plug and a regular type spark plug. Lay them on top of
the cylinder head and connect the spark plug cable to
first one and then the other. Spin the flywheel and notice
the spark across the electrodes. You will see that the
spark across the resistor plug will be much thinner and
makes less noise and yet we know that engines run very
well on these plugs.
Figure 32.
The magneto can be tested by placing the spark tester,
#19051, between the ignition cable and the spark plug as
shown in Fig. 32. Then spin the flywheel vigorously.
The spark should jump the .166" gap.
This test can also be performed with the engine running
but the cable should be shifted quickly from spark plug to
tester or from tester to spark plug. Damage to the coil
can result if the engine spins more than just a few
revolutions with the cable disconnected. This running
test should not be performed on the Models 9, 14, 19, 23
with the Magnematic ignition system.
Through the years the magneto systems on the various
Briggs & Stratton engines have differed somewhat in the
design of the parts. However, the basic principle of a
primary and a secondary circuit is used in all models.
Figure 33.
On small engines, be sure that the flywheel key is not
partially sheared as this can cause the timing to be off
enough to result in hard starting. Do not, however, use a
steel key. The soft metal key is used so that if the
flywheel should become loose the key will be sheared,
allowing the flywheel to shift and stop the engine before
any further damage occurs. Remember that the flywheel
key is a locater and not a driver.
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