The '85 SS (305) I bought came with Undersized pulley's- I guess it is supposed to give you more power. Unfortunately, my charging system shows a negative drain until I reach at least 1500 RPM. I had the alternator checked and it is charging properly. The alternator is a High Output ( 100 amp).

I believe the problem may be due to the pulleys, but I would like your opinions.
The Bottom Pulley is 5 5/8 In dia. I do have the original pulley- it is 7 1/4 In Dia.
The Alternator Pulley is 2 5/8 In Dia.
The Upper Pulley is 5 5/8 in dia.

Would replacing the lower pulley make a difference?
Is the Alternator Pulley the correct size?

My nephew put a 502 in a 67 Camaro with under drive pulleys had the same problem with the charging system , changed the pulley and problem was solved.