Porosity/pinholes in mild steel

I recently started to weld vertical up butt joint with tig the root run must be in tig and the cap in mig so after welding the root run it leaves a lot of pinholes so i would like to know what am i doing wrong? Thanks

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very likely, it is too much gas flow or too small a cup or some of each.

i have seen this before when using a small cup to get down close to the root...too much gas with a small cup causes air to be drawn in to the argon.