Make sure you don't have a bad spool of wire. I had a spool of wire at work that was rusty about half way through the spool and it was causing porosity. Now I have something like a woman's hair clip with a piece of Scotchbrite wrapped around the wire just before the feeder, so it will clean the wire before it goes into the feeder...it makes a big difference...it will keep the liner in the whip in better shape ...But like the others have said clean weld prep, make sure the defuser and the nozzle are in good shape, clip the wire before you start a new weld and check the gas flow
This happens to me all too often. I've been told that it is related to gas after flow. The holes that allow the gas to flow into shielded weld area can become blocked over time as the threaded collar begins to back out with use. See my pictures below. First one is the proper position with the holes clear and the second one is partially blocking the holes restricting the flow of gas after you release the trigger. So when you go clean the tip and tighten everything up it works well for a while and in time you get back to the same old slag bubble because the collar has loosened itself and is blocking the gas flow. I'm also told that you can install this collar backwards which covers the holes and restricts the gas flow.
Yeah..."porosity" or "slag inclusion"...
In any event, it indicates a contaminated weld site. Trace rust, grease, paint...even residue from a grinding disc...you have to make sure the weld site is clean.
Someone already mentioned this, but other causes/contributors can be a lack of shielding gas at the weld site, common if you are welding anywhere there is a strong breeze, as well as the tip of the wire. When you are tacking, you are going slowly, and each time you tack, you create a "slag tip" (there is probably a REAL term for it) on the end of the wire.
This burnt end of the wire is no longer clean wire...it's oxidized, and equivalent to mill scale on hot rolled steel. So, when you go to your next tack, the first thing the weld sees is contamination.
The remedy to that is to keep your MIG pliers handy and trim the end of the wire between tacks.