I have never worked on, not seen a carburetor that has adjustments under a welch plug.
All welch plugs that I have ever removed are simply meant to seal a chamber where two or more straight passages running at different angles come to a confluence at the exterior of the carb body.
Granted, when the plug is removed the passages revealed can be drilled for cleaning or to be enlarged but there is nothing adjustable in there.
Mainly the plugs are removed to clean the passages that are exposed. Drilling or punching a hole in the plug, which is soft metal, is a common way to remove them.
Once removed, the plug must be replaced with a new one which can be a PITA to find one of the proper size. Some rebuild kits provide them, some don't.
Since yours is a new carb, it is possible there is manufacturing debris lodged inside some where. I'd try dis-assembly and lots of poking and probing with thin wires and compressed air.
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