1992 Z24 Sputtering

I got a 92 Z24 i reciently changed the motor in it, 3.1L MP and here latly the car starts to sputter for no reason and all and dies, start it back up again and keep it going and it will work best kind, when she starts to sputter she'll drive alot of black smoke out of the exhaust, Any ideas?

If it's black smoke...maybe Piston Rings.

it will only put out a small bit of black smoke when the car sputters, other wise no black smoke will come out

There is a small crack in 1 of the coil, could this be doing it?