* 1983 240 turbo* the symptoms are -- down on power (it hesistates) and it doesnt always like to idle (sometimes it wont idle upon starting but i ease the gas down to idle speed and let off and it stays. sometimes when i come to lights or break hard it stalls, it fumbles with the rpm and most of the time recovers) black smoke is coming out the back (unused burning fuel) its not oil i know that for sure. when the car runs for about 20 minutes all symptoms go away and it runs fast and great. (except it still doesnt idle). my brother who is firmiliar with turboed cars seems to think it is too rich, and that i need to replace the o2 sensor. what else could it be??? please give me ideas.

there is only one o2 sensor, correct? right behind the turbo?