Any idea what causes this?

74 Dart 225. Seems like it is idling high but when I drop it into gear it brings the rpms down too low and dies if I don't keep foot on gas and brake. Also having trouble accelerating. When I give it gas it bogs down, I have to keep pumping gas pedal to get her to pick up speed. Car is 17 years older than me so I don't know much about how all that stuff worked back then