dt125a&b starter/generator

I am not familiar with the bike at all, but I do know a bit about electric motors and generators.

There is really very little difference between a DC "generator" and "motor". There is a stator that creates a magnetic field (either from permanent magnets or a coil with current flowing through it) and a rotor that turns inside it. The rotor has a "commutator" that switches between multiple coils as the rotor turns.

Connect it to a battery and the rotor will want to turn. Resist it from turning and it will draw a lot current from the battery. Let it turn freely and the current is very low. Force it to turn a little faster and the current goes to zero. Turn it even faster and it generates electricity instead of consuming it.

I am guessing that your need to know is because it isn't working.

If it doesn't work as either a motor or a generator then I would suspect that either the brushes are bad or a winding has burned out. Commutated motors really wear out brushes fast so I would suspect them first.

As the rotor turns and the brushes work there is usually a small amount of arcing. This can eventually eat at the commutator and create bad spots. Ending up with a dead spot on the commutator can keep the motor from starting if it happens to stop at that spot, or just really reduce the effectiveness of the motor or generator if it manages to keep turning.

If it works okay as a motor but not as a generator then I would look for a voltage regulator problem.

Rod