There are so many factors that can cause this, from a sooted up flue to oil pump problems. I think you need to call out an OFTEC qualified guy - he should clean the boiler flue and baffles, clean oil filters in the tank supply and pump, check the oil pump for delivery pressure, replace the nozzle, do a smoke test and finally a gas analysis when he is finished. If you can give details of the burner type it would help.
John (OFTEC), April 2009
