Furnace clean and tune. Cycled system. Checked heat exchanger. Fou;d all chambers have rust, but no visible cracks found at this time. Checked and cleaned burners and flame sensor. Ok. Found some signs of ware on igniter, but it tested ok. Checked and adjusted gas pressures to maximize efficiency. Found evidence of ventor motor being worn. Checked for levels of CO AND XS air in exhaust. Normal levels found. No CO found in airstream. System is worn, but operating normal at this time. Job complete.