You're not using flux wire are you? What is the regulator set at? When you squeeze the trigger, does shielding gas come out? In my Miller 135, under the side cover there is a terminal with two links- they can be turned one way or another. If I remember right, that's how you set it for flux wire, or for gas. But I can't remember for sure, and I don't have the welder here to look at it.
