The Yamaha uses a special type of magneto system with the "trigger" mechanism housed in that little gold box that attaches to the outside of the engine. For as expensive as that X%*?@ little thing is (Yamaha calls it the TCI module), it is surprisingly simple inside. The primary current flows through the wire coming out of the coil, into the TCI module, and grounds through the case of the TCI. (See Figure 2) That is why it is so important to be sure you have a good connection between the TCI and the engine crankcase. Be sure it is mounted securely, either directly on the engine or to something connected to the engine. The TCI contains a small circuit board with a few resistors and capacitors, and a good-sized transistor. The transistor is the heart of the system and it, in effect, "measures" the amount of current flowing through it. As the rotor moves past the coil, the voltage builds, reaching a peak when the magnets are just about centered on the coil. Just before this peak, the transistor in the TCI reacts. Think of it as flipping an "electronic switch". The circuit opens and, with no place to go, the voltage bounces back. That voltage spike in the primary windings of the coil energizes the secondary windings and that high voltage then goes in search of someplace to go. The path of least resistance is across the sparkplug gap to ground on the cylinder head. Bingo! The plug fires and so does your engine. The only variable that affects when the spark occurs is when the rotor/coil combination generates the correct current to open the transistor in the TCI. There use to be a significant variance between the older TCI boxes, measured as current required to open the transistor. But, curiously, these differences did not translate into any measurable timing or performance differences. The newer TCI boxes, standard with Yamahas and PRDs, have virtually no variance in current value required. Since the tech rules dictate that the ignition timing key and rotor are fixed, this means that, regardless of the TO value, all Yamaha KT 100s have about the same ignition timing, measured in degrees of crank rotation before top dead center.
Again, the principle is pretty much the same. The rotating magnet on the ignition rotors moves its magnetic field through the field surrounding the fixed coil. That induces a potential voltage in the primary windings of the coil. That potential causes a low voltage current to flow from the coil via the primary wire, to the TCI, and through the TCI to ground against the engine. When the current reaches a pre-determined value engineered into the transistor in the TCI box, that transistor opens and the current suddenly stops flowing. "flyback" effect causes a voltage spike to occur in the primary winding and that energizes the secondary winding. With many many times more wire turns in the secondary than the primary, the voltage potential induced in the secondary winding is much higher. High enough, in fact, to jump the sparkplug gap to ground itself, causing the spark, which is what we really wanted all along.
The critical factors are:
Solid connections between the coil primary wire and the TCI box.
Solid connection between the TCI box and the engine casting.
No shorts or other interruptions to divert the current from the TCI box.
This last point is important because it is how you hook up a kill switch to the Yamaha, if you wish to.
A simple wire from the connection between the primary wire and the TCI, running to a switch does the trick. Just hook the other side of the switch to a wire that is grounded and, when the switch is flipped, the current from the primary wire will take the path of least resistance and bypass the TCI box and go through the switch to ground. Bingo! No current to the TCI box, no spark.
Most of the Reed Valve 2 cycles in use today, including Gearbox engines, using some variation of the CDI systems first developed for Italian kart engines in the 70s. CDI stands for "Capacitive Discharge Ignition. Once again we have a rotating magnet and a fixed coil or "stator". In contrast to the Yamaha system, however, the control module is housed in the stator with the primary windings, and the coil is external. Those primary windings called the Figure 3 "charge coil" in the stator are mirrored in the external "ignition" coil, where they share space with the secondary windings. But also in the stator is the "pickup" or "pulser" coil. (See Figure 3) So, as the magnets in the rotor move past the pickups in the stator, they induce voltage in the primary windings of the charge coil in the stator.
When the control circuitry in the stator, triggered by a signal from the pulser coil, disables the current's path to ground, that current flows through the wires connecting the stator to the coil where it flows through matching windings. These, in turn, power up the secondary windings and the voltage is discharged across the sparkplug gap. There are important differences to note between these systems, whether they be Selecta, ltalsystems, or whoever. Some also include an external Capacitive Discharge module in addition to the external ignition coil. But in each case the stator-to-coil connection consists of at least two wires; a primary and a ground. This means that you don't have to worry about mounting the coil on the engine itself. As long as the wires reach, you can mount the coil wherever it is convenient. Often this means mounting it in a location more protected or less subject to vibration. The second difference, and the one that can make an important performance difference, is that using three sets of windings means that the voltage can be "stepped up" a bit more than the two set configuration in the Yamaha ignition. That can yield a hotter spark and more efficient and complete combustion.