I usually run stainless at 80-90 amps and purge at 10-15 cfh. What size wire are you using? Fit up the joint so you have the root gap and root face the same as the wire diameter for consistant results. For 2" sch 80, try 3/32" wire with 3/32" root opening (or root gap) and 3/32" root face (or land).

I use a 1/8" root opening with 1/8" filler wire. Prep the pipe with a knife edge. Set the purge at 15-20 cfh with a 1/4" bleeder hole. Start at 6 o'clock and do the "Hard side" first, that is the side where you are most uncomfortable for me(Being right handed) it is from 6 o'clock to 3 o'clock. Start on the tack(remember to feather the tacks)and once the tack starts to melt, add your wire keeping the wire in the center of the gap and minimize the ocillation of the tungsten. I use 60-70 amps for the root pass and I "walk the cup". Inspect that quarter of the joint to see if you have complete root penetration. The weld should be flat to slightly convex especially the further away from 6 o'clock you go.
The "stick-out" of the tungsten should be 3/8" -7/16" from tip of tungsten to edge of the cup.
Stainless tends to draw together so if you have a root opening of 1/8" all around , then after the 1st quarter of the joint is done, then it will be slightly tighter where your wire will not go into the gap, thru the pipe. This is alright if it is not so tight that a 3/32" wire will not go thru the gap. As I said, minimize the occillation of the tungsten.... this is the key to a good root pass.
You do not have to hurry, but be deliberate.....have spare sharpened tungsten and a grinder ready and your wire cutters handy. Do 1 quarter, then inspect, then do the next quarter (6 o'clock to 9 o'clock)....inspect. If the root openig is starting to close up (past 3/32" gap) the you need to thin-out the root opening where it is tight. Use a 1/8" grinding disc , but do NOT make the gap 1/8" wide, but thin the edge of the root so that with a little force , you could push the wire thru the gap(if you had to). Lay the wire on the root from the 3 (or 9 o'clock tack) and proceed to 12 0'clock. IF the root opening opens up and you cannot get wire into it fast enough then you are going too slow or you thinned it out too much. Finish the last quarterand you are done the root.
Let the coupon cool down completely before you start the fill pass. There should be no way to ruin the root pass (from suck-back) if you let it cool completely.
Weld on!