Lincoln Welder?

No, i'm not useing it for anything other then sheet metal (roof work), but it wants to burn through sometimes, or just leave heavy black soot, i'm getting sick of it, i'm only out 80.00 & thats with a 1 yr. warr., Harbor freight says they will take it back, its a Chicago welder, i'm just useing a standard style extention cord, so maybe thats my problem, i can't even make a pass, just a tack here & there, it really sucks & is very frustrating to say the least, it seems to do good at times, then crappy the next, i'm coming to the conclusion that flux is crapolla. I'm limited to the small 3 prong outlet, its at my storage.

What is a good extention cord to use? I'll look around for a used unit about 130 that can run off 115v.