Oil fired central heating boiler?

I recently part serviced a boiler which had not been seen to for 6 years. I cleaned the pre filter, pump filter and replaced the nozzle with one which i was assured was the correct one for the model. I cleaned out the fire box. It worked immediately giving better water temperature faster and came on lss often. A week later it started to produce a lot of black smoke although without apparently changing anything else. should I cleanout the flue or could the nozzle not be correct_
Bill Griffiths