An electric spark must be provided in a cylinder to ignite the fuel mixture. This is the function of the ignition system. The spark must be of sufficient intensity not only to ignite the fuel mixture, but also to ignite it at the correct time. A good ignition system which functions properly is very important.
In a small gas engine ignition system, the current starts in the primary circuit. The current is devloped through the armature and the magnetic portion of the flywheel. As the flywheel turns, a current is creates as the magnetic part of the flywheel passes under the armature. The current in the primary circuit is essentially a low-voltage circuit. As the breaker points open and close, electric current pulses through the primary coil. This pulsing current causes waves of magnetism to form. Each time the breaker points open, the magnetic field in the primary coil collapses, crossing the many turns of wire in the secondary coil. At this moment, a very rapid high-voltage current is crested in the secondary coil, up to 20,000 volts. This high voltage current jumps across the gap of the spark plug, igniting the charge of fuel in the cylinder during the beginning of the power stroke.