BBH,
Your evidence at this point is less than clear, so I'll answer the easy question first: If the gasket around the main passage plug leaks, you will experience a fuel leak, not an air leak.
My best guess as to what is preventing your engine from starting is that you have ignition problem, not a fuel or air problem.
If you remove the drain to the float chamber after cranking the engine with the choke closed for a few seconds (20 or so), the fuel that puddles in the bottom of the intake throat will subtract a bit of the fuel that is left to drain out after you remove the drain plug.
If you have a vacuum leak in the induction system (which is what I assume you mean by an "air leak"), it seems to me that you wouldn't have fuel puddling in the bottom of the intake throat of the carburetor. A vacuum leak sufficient to prevent starting should also preclude the puddling of fuel.
Don
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