1973 valiant 318 car stalls when compressor comes on

I even ran a separate battery ran the compressor power wire to the battery and a ground from the battery compressor had the car running and compressor kicks on but car stumbled even with the idle up put it in gear car stalls something has to be wrong with the compressor or it's overcharged I'm going to have to suck it down but can't do that nobody has a r 12 recovery system anymore this car was never retro fitted