1985 Other Oldsmobile Models Black smoke and jerks while try

When i start my car black smokes billows out of the tailpipe and if i dont maintian constant pressure on theaccelerator the car will die. I have to quick kick it into drive and hope i dont run into any stop signs cause the car will die. Once on the road, if i try and maintian a consistant speed it jerks almost violently and will lose speed. So I have to constantly accelerate and slow down to get where I am going. This is killing my gas mileage. Its a 1985 Oldsmobile Toronado (Caliente Package).