'87 stanza running rich/rough

[PLNCRZY]

Here is the story/symptoms

Today was very humid and cold.. freezing fog....about -10 celcius or 15-20 F.... anyway, got in my car, started just fine.. and I began driving to work ..(the car was not warmed up) about 2 blocks from home it began to run rough, would not accelerate very well, missing lots, spewing black smoke ... I drove 10 km/6miles during which time the car would stall at stop lights and I would have to keep the throttle up just to keep it running. however just as I got to work it began to run a little better (once engine was completely and fully warmed up).... and when I parked the car after 12 km/7miles the car was running just fine, idled well at 750 rpm etc.....any ideas as to what I need to do?