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It seems like the generator on my '49 Custom 8 doesn't provide any charge when the lights are on at idle. I placed an ammeter in series with the car's ammeter and drove the car for while and noted the charge rate with and without the headlights on, at normal cruising speed and at idle. With no lights on and cruising at 55 mph in OD there's about a 4 to 4.5 amp charge rate (a little high, but the battery, although new, shows a specific gravity at the edge of "good" and "fair" on the hydrometer). If I put the headlights on then the charge rate goes to about 2 to 3 amps, but when I come to a stop sign at idle the ammeter shows a whopping discharge of 20 amps! That doesn't seem right. When I do the same test on my '50 Ford, with the lights on and at idle, the ammeter still showed a slight charge. I can't find anything in the 1950 Motor Manual in the voltage regulator section that addresses this. Anybody have any ideas as to what I can adjust, or do I just need a new regulator? Thanks.

Generators typically have little or no output at idle. You might check your idle speed, perhaps its abnormally low?

But, with a battery in "fair" condition by hydrometer, I'd expect a much higher charge rate at highway speed in daylight, 15 amps at least. So I suspect you have an issue with either the regulator or generator or both.

If you momentarily ground the generator field terminal and increase engine RPM and a very high charge rate (as high as 30 amps) results, you've narrowed the problem to the regulator. Remove the test jumper to ground immediately after you've completed the test.

I assume you have a "problem" with it and not just out testing for the curiosity of it?
Not sure I understand how you're measuring, but I think you're not measuring full amps if using the "gauge" leads. That's not really an "ammeter". It measures relative direction of current flow. A coil creates a field as the current flows and the magnetic needle moves with the field polarity. More current stronger field, more deflection. When needles in the "center", it's supposedly fully charged up but can still be drawing low amps. I think you have to measure on the output lead. If you have a clamp type ammeter it's easier than putting DVM in series.
I recall that '49 gen is rated 30 or 35 A. So if pulling 20, not much left at idle. If you need more power, then ether change the gen or have rewound. I think there's an amp v rpm chart, in the shop manual, I seen it, not sure where. Maybe Delco?
Take to the local parts store that has diagnostic tools and let them put their battery machine on it. They can test 6V.
Or an auto electric shop and let them test it all. Some charge, some don't.
Another thing to do is have someone pedal the gas while parked in neutral and observe the lights from the side (safety) They should not go dim at idle. A small difference is ok. I'd suspect the battery if dimming alot.