Motorcycle Repair: Charging System, heading in the right direction, starter bike

Question
I have a 1981 Honda CM400C.  I purchased the bike recently for my first starter bike and the headlights do not always work.  One time I will ride it and they will be on the next they won't.  Sometimes my battery is so drained it barely starts the bike.  Once the battery was dead and I had to charge it.  I looked into possible solutions and I got advised to check the battery, stator and regulator.  I haven't got a chance to check the regulator yet, but when I put the multimeter on the battery while running, it would stay constant at about 15 volts, and when revved the voltage would slowly decrease instead of increasing. When I was trying to get the plug out of the stator like they said, it stopped half way and got extremely loose like something on the back came off and then when trying to put it back in it doesn't want to seem to catch anything.  My first question is how to get the plug on the stator back together, and my second part is am I heading in the right direction for what to check.

Answer
Sorry for the delayed response but I'm confused with what you are describing.  I think the drop in voltage is due to the fact that the alternator isn't charging.  What you see initially is the surface charge and as the demand on the electrical system increases the alternator can't keep up and the voltage drops.  Have you checked the resistance between the three windings on the stator? They should all be the same resistance and none of them should have continuity to ground.  If the stator checks out okay, it's probably the regulator since that's about the only thing left except the flywheel.  I have seen the magnets go weak before but I don't know of a test for those.  I usually replace those last after I've checked everything else.    

I can't figure out which plug you are referring to in the stator?  Are you talking about the bolt holding the flywheel to the crank? There are 4 bolts holding the stator to the crankcase but I don't recall any plug.  If you can clarify what we're looking at I'll try to help.

Regards
Rich