TM 5-4240-501-14&PTHEORIES OF OPERATIONIgnitionSpark plug cables are molded into the coil so thatmoisture cannot short out the spark as could happen onolder coils that had an open connection between coil andspark plug cable.We would like to point out that at one time somemechanics would try to judge the condition of themagneto system by the brightness and the noise or"snap" of the spark. This is not a good criterion as youcan quickly demonstrate by using a resistor type sparkplug and a regular type spark plug. Lay them on top ofthe cylinder head and connect the spark plug cable tofirst one and then the other. Spin the flywheel and noticethe spark across the electrodes. You will see that thespark across the resistor plug will be much thinner andmakes less noise and yet we know that engines run verywell on these plugs.Figure 32.The magneto can be tested by placing the spark tester,#19051, between the ignition cable and the spark plug asshown in Fig. 32. Then spin the flywheel vigorously.The spark should jump the .166" gap.This test can also be performed with the engine runningbut the cable should be shifted quickly from spark plug totester or from tester to spark plug. Damage to the coilcan result if the engine spins more than just a fewrevolutions with the cable disconnected. This runningtest should not be performed on the Models 9, 14, 19, 23with the Magnematic ignition system.Through the years the magneto systems on the variousBriggs & Stratton engines have differed somewhat in thedesign of the parts. However, the basic principle of aprimary and a secondary circuit is used in all models.Figure 33.On small engines, be sure that the flywheel key is notpartially sheared as this can cause the timing to be offenough to result in hard starting. Do not, however, use asteel key. The soft metal key is used so that if theflywheel should become loose the key will be sheared,allowing the flywheel to shift and stop the engine beforeany further damage occurs. Remember that the flywheelkey is a locater and not a driver.1714
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