My boiler suddenly started producing a lot of black smoke about a month ago, coupled with noisy firing. It had not been touched and had been running beautifully since it was installed last September. It was very sooted up inside, so I cleaned it out and that seemed to cure it. Last week it happened again. It was dirty, but not too bad. All the advice I could find suggested the nozzle. I change this and checked the oil pressure as well (OK) but it has made no difference. What should I be looking for next?