What welder are you using? What are the settings? What type/size of wire? Are you triggering the welder on/off, or are you trying to run a bead with the trigger pulled? What type of gas are you using? Is the old metal ground clean and free of rust and debris? Is the joint fitting good and tight? Those butt weld clamps with their .040 gap don't work too well. You can put copper back-up behind the weld and that will help.