The ignition system of motorcycle accessories engine generally adopts the flywheel rotor type, while magnet and flywheel are united into one and form as a part of the engine flywheel, and permanent magnets rotate together with the flywheel and form a magnetic field, then there will be induced current in the fixed coil, at the same time play a role of mechanical energy storage and release. Four-stroke engine use non-contact ignition system, the trigger coil of magneto is at the external of the flywheel rotor, where there is a permanent magnets, with the rotor rotating, trigger coil will induce pulse current and provide trigger signal to electronic ignition device (CDI).

In order to ensure that the ignition system has a strong ignition trigger signal, while it will not interfere trigger poles and flywheel rotor, so there is trigger gap around 0.40 mm to 0.80 mm between the trigger pole and the flywheel rotor. In general, the gap can be adjusted by the mounting hole or oval hole on trigger pole. Due to the error at manufacturing, or reassemble after repair, or trigger pole wear and tear, the ignition trigger gap may get bigger, thus affecting engine ignition performance in different degrees. Besides, there is also air gap between cylindrical stator and flywheel magnetic, under normal circumstances, unilateral air gap should be no greater than 0.75 mm, in order to ensure the magneto ignition and charging system work normally.


For a 125 type motorcycle, driving more than 43000 kilometers, after the stator component was changed for the magneto fault and then electric start can’t work while the pedal start is normal. Then, repairman will check both static and dynamic of battery voltage, which turns to be normal, overrunning clutch, start circuit and relay are also normal, then check the starter circuit, carbon brush and so on, they are all normal. Remove the spark plug, put it on the high-pressure damping cap and do electric start check, the spark is dark red, and after changing a spark plug, electric start is still not working. Then you need to replace the stator coil, and remove the ventilation cover, check the trigger gap between flywheel and trigger pole, there are unexpectedly about 2.0 mm. Realigns the trigger to standard 0.50 mm, reinstall the parts, then electric start can work successful now.

Another 125 scooter, due to loss of excitation with the magneto flywheel rotor, vehicle cannot be started even after replacing new parts. After several check, the problem turns to be the flywheel rotor wall, whose diameter is too large and exceeds the specified value between the stator cylindrical. Replace the magneto flywheel, check the gap between stator cylinder and the flywheel rotor wall, make sure it is less than 0.50 mm, assemble the dismantled parts, start the engine, then the failure is solved.

You can be seen from the examples above, repairman didn’t adjust the trigger gap according to the rule when changing the stator component, the ignition trigger gap will be too large. When applying the pedal start, because recoil inertia is big, it can barely achieve start speed; and when electric start, because starter speed is relatively low, the large gap will reduce the pulse signal, the spark is not strong enough to start. And the other motorcycle can’t be started because the air gap is too big between cylindrical stator and the flywheel rotor magnetic cylinder wall, which will seriously affect the ignition performance of magneto.