Motorcycle Repair: 81 650 special / stator and electrical, stator coils, regulator rectifier

Question
I have owned an 81 and 79 yamaha 650 special for years and last summer both came down with the same problem.  The stator doesn't seem to be charging the battery in either bike.  I've diagnosed with my volt meter and got readings that didn't make any sense according to my Clymer manual.  New Batteries go dead in 2-3 hours and amps seem to "dim" at stops or low rpm.  They both have about 20K miles and have run really well for many years.  Does this sound like I need new brushes/coil to you?  If you have any advice/info that would help it would be greatly appreciated.  Those two bikes are my babies.  Thanks Falcon  

Answer
Hi Tom.

Even though both bikes are well under 50,000 miles, they are both over 20 years old.  so it is very likely that the stator coils are both bad.  If, however they are producing enough power at the coil's connections to the regulator/rectifier, as per the specs in the manual, then there could be a short in the wiring between the stator and the regulator/rectifier or between the reg/rec and the battery.
 You could have to replace the brushes, but they aren't known to go bad too often.
 There is also the possibility that the reg/rec is bad. Test it to be sure.

You should check the battery with a hygrometer, too.  It will tell you if the batteries are still any good.  If they are 2 or more years old, then replace them anyway.

Start with that and let me know what you find.

Good luck and ride safe.
FALCON