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most likely it is either the coil, or spark plug cap. i have seen many spark plug caps go bad in the 35 years of repairing small vehicles like this. a quick way to check that (in your case) is to run the engine until it dies, then just remove the cap, and push the wire onto the end of the spark plug. if the machine runs fine, there is the problem. funny thing is, a bad cap will eventually take out a coil. as far as how the machine makes spark, as the engine turns, it rotates a flywheel. inside of the flywheel there are magnets. on the side of the engine under the flywheel, there are a series of coils called a stator. as the flywheel magnets go past the coils, it creates a magnetic field. that creates electricity. that current (the part for the ignition) is sent to a capacitor inside the cdi box. when the trigger coil (more magnet/coil stuff) sends voltage to a different section of the cdi box which sends the voltage created by the stator to the ignition coil at the exact time it needs to be there. there is also a "kill" section in the cdi box which either grounds out the spark, or interrupts (keeps) it from reaching the ignition coil. trigger coil voltage is usually in the 1-2 volt range (ac), the ignition voltage from the cdi box (same as from the stator) is normally in the 100 volts range (ac). the ignition coil is simply a step up transformer, bumping the 100 or so volts to around 15,000 volts. the spark plug cap has a resistor in it, so it minimizes the amount of radio frequency emitted by the ignition. that for most of us means nothing. but if you happen by a work site using high explosives, means life or death! that radio frequency emission can set off a blasting cap, and thus as much explosive as the crew is using.

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