THIS COULD ONE OF A FEW THINGS, ELECTRODES MAY NEED REPOSITIONED, THE NEW NOZZLE MAY BE FAULTY, AIR FLAP MAY NEED ADJUSTED, AIR MAY BE GETTING DRAWN INTO THE OIL SUPPLY ETC. A SMOKE TEST AND A FLUE GAS ANALYSIS WILL PROBABLY DETERMINE THE PROBLEM BUT ANY WORK ON AN OIL FIRED BURNER SHOULD BE CARRIED...