Recently, my 1971, 240z has been running funny. I go to start it, choke fully on, and it seems to run fine at a high idle, and when i rev it. If i completely release the choke, the car dies after about 30 seconds. Earlier today, i tried to drive it, and its completely powerless. The car is very sluggish, and i barely made it back into my driveway after only taking it around the block. The exhaust is a very dark grey/black color and smells very rich. I cant seem to find the problem. If you have any idea of what this could be, let me know. Thanks