You're referring to a burn through. It means that you are trying too hard to "consume your tack", and are basically burning through an existing weld. All you want to do is tie in good to both sides of your tack. Probably a little less grinding near the center of your tack would help. If you grind too much on your root, you are opening up the possibility of a burn through. A little less grinding, and more heat, maybe..............hope that helps.

  As for making the tie in to the tack try feathering the tack a little deeper to allow you to reconsume the edge of the tack as you travel onto it, also once the keyhole closes add a little pause and build up a little more metal in the tie in area. Suck back is caused by traveling too quickly or not adding enough metal. When it comes to installing consistent roots only practice and experience will get you there. Try installing your tacks at 3o'clock instead of from the top to prevent heavy push through and don't get in a hurry welding off of or onto your tack. Good luck.

For 2" sched 160 make your tacks small only about a 1/2" long if that.  Bead tie in's are all about how you grind and how fast you move once you burn into an existing tack or bead. Give yourself enough on the bottom to fire up on and then start pushin in to blown through.  Make it thin so you blow through for sure.  You'll know your getting thin when the tack starts to change colors on ya.  Watch close or you'll grind through.  Come up on your 3 o'clock or 9 o'clock tack and consume it.  Then go to your opposite top quarter and weld up to the top of the pipe and consume your top tack.  Now go back to the bottom and grind or feather that tack back so you start blowing through where you first root pass started burning in.  You'll need to feather the side tack now like a ramp so you'll burn the first part of the tack or bead away and as the tack get thicker you won't blow through anymore. (that where you start to speed up) . As you start getting into the thicker part of the ramp start pulling out slowly and speed up till your not even depositing any weld metal onto the existing bead. (make sure you fill the ramp up though)  That is how you keep it from sucking back.  If you don't put a big root on the inside it won't be as noticeable when you do suck back a little.  Don't get me wrong, you want to put enough in there in case you do suck back so it doesn't go below flush.  Hope this helps!