There should be a paper gasket between the tank bell and the crankcase and a small rubber one in a groove around the center tube between the bell and the backplate.

But, and sorry if this sounds silly, it sounds from your description that you expect the engine to start and run without touching the needle valve at all. It's normal to richen it up by 1/2 to 1 turn for starting and then lean it to the peak once running.

Like I said, pretty silly but I figured I'd mention it just in case.

If this isn't the case then I'd suspect crud buildup in your needle valve metering area or in/on the reed.