Well, you'll have to start checking and testing things related to the charging system. Start with the alternator brushes. They're considered a wear item on these bikes and once they wear too short, charging will drop off or even quit. New, they're about 14mm long. The wear limit is 7mm and if they're originals, there should be a wear line etched in the side of the brush .....
You can remove them to measure their length but it may be easier just to pull the alternator off. The spring on the brush will force it out of the holder enough so you can see the wear line .....
As they wear, the carbon that rubs off them turns the slip rings they bear against black. If they get too dirty, you can start losing contact. Maybe that's the issue. You can clean them with chrome cleaner. Here's one cleaned, the other not .....
..... and then both cleaned .....
If the brushes are OK then you'll want to test the rotor. To do that, you measure the resistance between the two copper slip rings. It should be 4 to 5 ohms. You can't have the brushes in contact with the rings when measuring so while the alternator housing is pulled off is the perfect time to do this test (and clean the rings of course).
So, do these checks then get back to us. If they check out then it's time to move on to the regulator and rectifier. If one of them proves to be the issue, we can provide info on good, cheap replacements.
