Furnace clean and tune. Cycled system. Checked heat exchanger, ok. Checked, removed, and cleaned burners and flame sensor. Both were very dirty and rusted. Cleaned as best as I could. Checked igniter, tested normal. Checked and tightened electrical connections and clamps. Cleared drain line. Checked and adjusted gas pressures to maximize efficiency. Preformed combustion test and slightly elevated levels of XS air were found . No CO found in airstream. System is operating normal at this time. Yearly maintenance is recommended to ensure proper operation. Job complete.