I have a 1997 1.9D Polo, it runs brilliantly apart from 1800 to 2200 rpm where it smokes very badly
When it does this it makes a rattling noise and it would be as if it was pinking if it was a petrol. When it hits 2200RPM it starts pulling very well and before 1800 it runs perfectly. The smoke is black and smells strongly of diesel.I initially thought it was an injector problem so put a new set of injectors on it. This made no difference.
At this point i was given another car with the engine running perfectly (well no smoke) but had a lot of oil in the water so didn't bother swapping the whole thing. Instead i swapped the pumps over the original was a lucas pump and had lots of wires going to it and what seems to be a small ECU on the bottom of it. I assumed that since it was lucas the electrical components would be faulty
, thinking that the advance was stuck at a certain point. The pump I put on is a bosch one and only has one wire for the cut off valve. Much better 
Changing them over made no difference at all. I reset the timing again and it seemed to make it a little better (less smoke) but still the same problem at the same revs.... well it my have moved up from 1800 to about 1900 before it starts doing it, but that could just be my imagination after spending days trying to fix it.
Cheers
Stu