I have a 75,000 btu input Arco Air gas furnace that was converted to LP gas when it was installed in 2000 that wasn't working properly. I went out on the call and only one of the burners would light. I noticed that the three burners were all carboned up. I cleaned the burners and the thing lit all three of them. As it ran the flames would occassionally creep back into the burner assembly. But it wasn't really a lazy flame like a venting problem. It would just creep back into the mixing chamber periodically. I checked the gas pressures inlet and outlet of the gas valve both were good. ( 12 in wc inlet and 10 in wc outlet). I disconnected the vent pipe but it did not mater. I am suspecting maybe a small crack in the heat exchanger just enough to effect the draw of combustion gas into the heat exchanger but I am not sure. I was hoping that you guys might offer some insight.