The goo inside your burner is unburnt fuel, and this is due to incomplete combustion for some reason. Maybe the boiler flue baffles need cleaning (first thing to do and essential). Once this is done and you are sure that everything is clean, its time to have the burner cleaned and set up properly. A competent person will check/adjust the pump pressure, then do a smoke test on the system. If that is clean enough, he should then do a complete gas test, and its only in this way that he will be able to balance the amount of air going into the burner to achieve something in the region of 11% CO2. Once this is done, no more soot for a year or so. The burner nozzle should be replaced at the same time for good measure. I assume that you are burning kerosene, not heavy oil which isn't suitable for domestic systems.
John, April 2009
