Porosity problems.

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Hi folks  have recently bought a budjet tig welding set £300 off ebay recently which has ac/dc function (no pulse) and have had mixed results with the unit. Its a chinese machine and have so far managed to do a couple of aluminum jobs with satisfactory results on ac setting. I have been practicing trying to do pipe butt welds ie root,fill,cap on carbon steel (dc )but keep getting gas traps & porosity appearing frequently in the welds. my metal is clean the preps are sound and my gas flow has been checked properly. Does anybody know anything about this problem that could help or know if it could be connected with an electical fault or have I just landed myself a cheap & nasty unit that is probably unfixable. I am just new to this website & would be really grateful for some advice.
Reply:Few questions first,What kind of shielding gas are you running?Are you running DC negative?How much pre flow and post flow are you running?Be sure your filler rod is also clean and somewhat old and oxidizedMechanical Engineer
Reply:TIG requires a deoxidized filler, ER70S.You can't use O/A filler, RG-45 or RG-60.9-11-2001......We Will Never ForgetRetired desk jockey. Hobby weldor with a little training. Craftsman O/A---Flat, Vert, Ovhd, Horz. Miller Syncrowave 250
Reply:Check for leaks or where air may be getting sucked in, venturi effect, or your gas may be too high, contamination, start with all suggestions by membersCheers
Reply:Thanks Billy thats some good new advice . Leak check is next on agenda. Will post a thread as to how it goes.Cheers.B
Reply:Hi BLUE2KSS. My shielding gas is pure argon from BOC. Polarity is Dc negative pre flow 1.5-2.0 secs post flow 3-4 secs slope down 2 secs.The wire is Brittish Oxygen company own cheaper stuff code 35461 ccms which is not the best quality I know but  have used it with decent results in my workplace on a better quality migatronic machine. Next time I will get some better ER 70s wire. Thanks for your reply.B
Reply:Thanks Craig Im going to get some good quality ER70 S next time.B
Reply:since he did not report blowing the snot out of his tungsten and gas cup we can assume he is dc negative..
Reply:Is it windy where you are welding? Shielding gas could be blowing away.- If you can jump across it you can weld it!  - anonymous old boilermaker
Reply:No its inside the workshop. Thanks B.