Upon arrival unit was on but was off on reset. Pushed the reset button and the unit started up but was rumbling slightly. Checked oil flow at the burner and had a clean and steady stream. Checked the ignition transformer and it was good. Checked the nozzle and found carbon and oil buildup on the nozzle and in the blast tube. Replaced the nozzle and cleaned up all carbon and oil residue. Started the unit and performed combustion and smoke tests. Unit is working properly. 7.1%o2 1ppm co 0 smoke