My suggestion would be to leave the welch plugs in place, but seal the edges with fingernail polish. I believe there are sometimes check valves and sometimes screens behind the welch plugs, but I'm not really sure because I leave them in place. I get the impression trying to remove the welch plugs can easily damage the carb, so I rebuild the carb without removing them.
I can tell you I've rebuilt a carb and it still wouldn't run right - was advised to go back and seal the welch plug with fingernail polish - did that and it ran great.
If you want more detailed advice, go to Arboristsite -there are guys there that know way more than I do.
Good luck, I'm definitely still learning myself.
By the way, for some saws, a new carb can be almost as cheap as a rebuild kit.
In spite of the fact that many Stihl parts are priced very high, I bought a brand new carb for a MS 250 chainsaw for $17.50. The rebuild kit was $12.95. Pretty obvious why I bought the carb instead of the kit.