IGNITION BASICS

A magneto is in fact a combination of a simple dynamo, the contactbreaker mechanism, the low voltage coil and the high voltage coil. To understand how it works, I can better start with the battery ignition (fig 1). As you see, if the points are closed, current can flow from the battery pluspole through the low-tension coil and the contact breaker to ground, and thus battery minus pole. This is the lowtension circuit. Nothing really happens yet, except the coil heats up. Now if the cam turns round, at some point the contacts will open and the current is cut off. This sudden change is transformed to a high voltage spike in the high tension coil. That high voltage can now jump accros the sparkplug, and thus ignite the fuel mixture. This simple description leaves a lot of questions open. I will answer some of them, as far as they are of interest for a motorcycle mechanic. First I need to explain some of the electro-magnetic theory.