Are you using a foot pedal or a switch? Welding thin stuff is really more about technique than equipment. Small tacks first about an inch or so apart, then skip weld it fully. The point is to keep the part from getting too hot. Copper heat sinks near the weld help alot. Use the thinnest filler rod you can get, and just get a puddle started on both pieces and dip the rod guickly to fill the puddle, and move on. Takes practice but once you get the rythem it goes easily.

Steve

Ps It sometimes helps have your shielding gas directed at the back of your weld, for instance filling the muffler with argon by tapping of the supply line from the torch.