Troubleshooting

So I ride an old, decommissioned police bike...among all the switches is one that turns the headlight on and off...
I've found that when the headlight is switched off, my bike idles great, but as soon as I switch the light on it sputters and eventually dies.
What's the deal? How can I remedy this? Is it the charging system?

Sounds like the headlight is drawing more juice than the bike can make at idle.
Does it only falter at idle? If so, then maybe your battery's not holding a charge. The generator doesn't send much current to the battery at idle, and if your battery can't store it up, then there won't be enough juice for the coils. At about 3000 rpms, the problem should go away, if your generator is working right.
...OR...
Your headlight may be drawing more current than it should due to bad/dirty connections. Or you may have a super brite upgraded headlight bulb that draws more current than the original bulb. The original bulb draws 35W on low beam, and 50W on high beam, but upgrade bulbs can draw 85W, and they can tax your charging system, especially at idle.

It goes away as the rpm's increase, so my normal performance isn't affected, only idle is fluttery. My battery is brand new and holds a great charge, but I don't know anything about the bulb that's in there

I wonder if the charging sysetm is working right, or you still have something going to ground from the old system

Lee,a standard bulb will be 55/60 watts.
You should pull the bulb out and have a look at it,to see what wattage it is,to be sure.
I know others have,and are using a higher wattage bulb,and with some rewiring and a relay,it can work.