That is a welch plug covering the transition ports for the idle side of the carb. I take a small drill bit and carefully drill through it but be careful not to let the drill bit drop in too far otherwise it will damage the little holes behind it and may not run right afterwards. insert a punch into the hole that you created and pry the plug out. clean with a good spray type carb cleaner and a replacement plug comes in the carb repair kit. just set the new plug back in place and flatten with a flat punch large enough to cover the entire with of the welch plug. then seal with fingernail polish. removing that plug is part of cleaning the carb on tecumseh engines.
