My charging system works, BUT, the battery will drain when I turn on my headlight. If I then turn off my headlight, the battery will charge right up?????

What bike are you working on? The 125cc AT or DT series with electric start are the only models where the headlight on might drain the battery significantly.

If it is an 12v electric start model, the manuals in the Tech Library have great troubleshooting sections.

If it's a 6v magneto model, it could be someone made wiring changes.

You didn’t say if you started with a new, fully charged battery...

Previous post suggests AT3.

I'm guessing it might be a 6volt model & if so, then much like--think it was Mauser up there said, reduce headlight wattage & also tail light if magneto powered like the headlight. & check the brake light filament isn't on through crossed wires rather than tail light lower wattage part of the bulb. If bulbs are above original wattage & leaves less voltage to charge the battery. Charging is never great with lights on on those early systems. If its had a regulator added & providing the battery is ok & up to level if a lead acid battery then disconnecting the single wire regulator should give more back to the battery at revs . One way to find out--try it. But all that woffling is providing the bike is 6 volt. If electric start 12V then forget what i said haha. MY old AT1 magneto lights has [by memory] a double yellow connector under the tank to hook the green into to get more daytime charge, but that's not your bikes problem.

I start out with a fully charged battery. I can ride all day without a problem, BUT if I turn on my headlight within a couple of hours the battery will drain to the point that I have to use the kick starter.. to re-start. After I kick start it and ride about 10 miles with the headlight OFF the battery is charged enough to electrically start the bike. Sitting at idle with the engine running and the headlight turned on it will get brighter when I rev the engine???

Oh, ok, from that i guess it's a 12 volt generator system if you have to kick start when near flat. I'm out of ideas except it sounds like the regulator needs adjusting. But then again is the battery 90% stuffed & not taking the charge provided. I gather you can use the starter first time then its down hill after that--with lights on. Are the carbon brushes ok. If they're worn right down there'd be a point where the starter wouldn't work, or even try to work, & the generator wouldn't make the lights go brighter. Need a video if possible. Check the loom earth is attached to the frame.
Reading again it just sounds like it needs the regulator cranking up to about 14.5 volts. They're a bit of an electric brake, so the higher it charges the more it'll try to hold the motor back, but 14.5 is about right by memory.

So it is a 125. First question is if the headlight bulb is stock wattage? I think they are only 25 watts.

I'm sort of guessing that if its 12 volt & electric start the headlight will likely be a 35 watt sealed beam if you're in the US. But regardless, the generator just compensates for the draw & keeps on pumping at 14+ volts above idle. How about a pic of the bike from the left side, & a pic of what battery is in there--big 12 or a toy size 12. Need more info--'don't we guys'.

I looked in the specs and the US 12v 125 had a 25/25 watt headlight through 1973... the generator was changed slightly which seems to coincide with 1974 coming with a 30/30 watt headlight.

The generator regulator does increase output... but the generator only has so much output available... if someone installed a common 55 watt headlight on a system designed for 25 watts it's likely that even at full field voltage from the regulator the generator at max output could not keep up with the electrical demand and the battery could slowly discharge over a couple of hours.

So headlight wattage would be the first thing I'd check... and yes, we need more info. Still don't even know the year model.