There's a gasket under it, so that one is simply a plug. Tighten it. Some Zenith carbs had adjustable main jets, with a knurled knob in approximately the same position as the plug yours has. Your carb probably has a fixed-size jet that's accessible by removing the external plug.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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