The sensors are simple single wound coils with fewer windings than the "charge coil". The sensor coils only develop a few volts when the flywheel magnets pass by them. These low voltage signals are what operate the SCRs inside the powerpack telling the powerpack when and what coil to send the primary voltage (stored in the powerpack’s capacitor) off to.
Most of the two cylinder engines have only one sensor coil and are fired by the outer flywheel magnets. The opposing magnets create sensor voltage in opposite directions, so the powerpack knows which SCR to fire by detecting the polarity of the low voltage sent by the single sensor coil. The three cylinder and larger engines (along with some of the 40-50 models with large alternators) have two sets of magnets on the flywheel. The outer magnets which are used for the charge coil(s) and alternator windings, then there is a magnetic ring on the inner flywheel hub which is used to fire the sensors.
The timing is controlled by rotating the sensor(s) on the mag plate, much like a conventional universal mag set up. Needless to say, the flywheel key must not be sheared or the timing will be affected. The tricky part of this ignition system is a sheared flywheel key will NOT affect the timing readings you see on a timing light. YES, the timing is definitely out of whack, but you won’t see this with a timing light. I know this is tough to fathom, but it is the case. This is why checking flywheel positioning with the TDC tool is a step that should not be ignored.
Later model engines introduced another pesky problem. The sensor magnet ring was glued in place, instead of being cast in place, another killer cost cutting technique. These glued in place sensor rings love to come "unglued", causing the timing to go crazy, along with the engine. This problem will show up on a timing light.
