Remove the tin shield around the flame. Be sure the fridge is off and the burner has cooled. Cover the burner so particles , dirt, rust, etc, cannot enter the slots on the burner cover. Get on the roof and remove the fridge vent cover. It will be necessary for you to remove the baffle in the boiler tube. It is on a wire anchored at the top of the boiler tube. Unhook it and just pull it out. Don't forget to put it back when cleaning is done.. Ideally a 10 gage shot gun cleaning brush just fits the boiler tube. A 12 gage will do. run the brush down in the boiler tube and clean the rust flakes out of the boiler tube. The cover below hopefully keeping any junk from entering the burner. Once done with this go back and go back over all the stuff you cleaned previously. DONOT put anything thru the gas orifice to clean it up. Instead soak in denatured alcohol then blow everything clean and dry with compressed air. If you do not have access to compressed air go to an electronics or computer store (RADIO SHACK) and get a can of clean instrument air. Since you are the second owner and the first owner had the problem chances are good that this has been building up to the present problem and this will hopefully clear it up.