A starter-generator is one electrical machine on the end of the crankshaft that works as the electric starter and then as the generator. It was used on some small bikes of about 100 to 185 cc from the 1960s to the mid 1970s. It has an armature with a commutator, carbon brushes, and field coils in an outer yoke. The ignition points are often mounted on the same plate.
What it does
When the starter button is pressed, battery current flows through the series field winding and the armature, and the unit turns the engine directly, with no reduction gears. Once the engine runs, a mechanical regulator box switches it over: the shunt field winding is fed, and the spinning armature makes direct current to charge the battery and run the lights. The regulator box also holds the cut-out that disconnects the battery at low revs so it does not drain back through the generator.
Signs it is worn or failing
- Button gives a click but the engine turns slowly or not at all with a good battery.
- Battery goes flat while riding, or the charge warning lamp stays on.
- Battery boils and bulbs blow: regulator not controlling output.
- Heavy sparking or burning smell at the brushes. See Starter-generator bike will not start on the button or will not charge.
When to replace it
Brushes are the usual wear part: replace them when short or chipped. Replace the armature or field coils only if a test shows a short to the body or an open winding. Whole units are scarce and can cost more than an old small bike is worth.
What to match when buying
- Brushes: size, shape and lead type. A larger brush filed down to size can work when the exact one cannot be found, as long as it slides freely and presses fully on the commutator.
- Regulator box: it must match the generator (voltage, terminal layout). A regulator from a different system will not work.
- Used parts from a donor bike of the same model are often the only source.
Fitting notes
- Test in this order: battery and connections, brushes, generator output apart from the regulator, field coils, armature, regulator. See Test a starter-generator (dynamo) on an older small bike.
- Disconnect the battery before removing the cover or touching the regulator.
- The armature usually comes off the crankshaft with a puller and is located by a key; refit the key before the armature.