Someone else posted about this a while back, but dang if I can't find the thread!
Some of he regulators have a black wire that's not found on other regulators. If I remember right, that black wire is an extra ground, but it's for voltage sensing. It's supposed to be connected as close to the negative cable from the battery as possible.
On a voltage regulator, it's internal voltage reference in always in relation to ground. if you have high current draw or any corrosion in the connectors, its ground reference can actually "float" slightly higher than true ground. As such, the voltage regulator always puts out power voltage than it should and that leads to problems.
This black wire (and someone please verify me on this, because I'm going from memory here) is connected to the frame where the negative from the battery connects. This allows the regulator to get a "true" ground reference.
Currently Running Stable:
2010 Honda NT700V
1986 Shadow 700
1986 Honda Trail 110 (Postie Bike)
1987 Honda Rebel 450
1973 Honda Mini Trail 50