I built a gas furnace to melt metal,but I'm having a problem with the flame.I'm using my air compressor to blow in air,the flame does good when the lid is open but when the lid is closed the flame dies down a little and turns blue. PLEASE HELP ON HOW TO GET THE RIGHT FLAME. THANKS

This sounds more like an engineering question than one in chemistry.
There aren't many detals provided, but maybe the closed lid , coupled with the compressed air, gives a back pressure that decreases the flow of gas.