You have a couple of things to consider on these alternators. I am assuming your using an old style 10si Delco with built in self exciting regulator. These regulators are an aftermarket product and there are several companies making different versions of them with different cut in settings. The self exciting regulator is offered in a low rpm cut in and it sounds as if yours does not have that particular regulator. This regulator will cut in when rpms hit around 300-400. Once this happens you should be good to go. If after that your still dipping down into the discharge then it sounds as if your drawing more amps than your alternator will put out at idle. This should not be and you could possible have internal issues with the alternator. On a low rpm application a smaller pulley will increase your cut in time as it will increase the rpms of the alternator, but its not necessary if the alternator is built correctly. I use a regulator that excites instantly, as soon as the alternator starts turning regardless of the rpms and I always use an alternator that is 55 amps plus, personal preference. If your going into discharge at idle with lights etc. on then something is wrong, you could get better performance than that out of the original generator.
Have the alternator tested at a local shop and they should be able to tell you if you have diodes etc. that may be bad and what the under load amperage output is.
