I would first find the source of the air in the fuel line. There should be no air visible in the line from the the needle valve to the carb.

The new engine should be run rich during break-in so you will see some fuel coming out of the muffler although it shouldn't leak out of the seam where the two halves are joined. Mine did and I removed the nut from the end of the muffler, tightened the screw and then replaced the nut. There *are* threads in back half of the muffler. The nut just 'locks' it.

Once these are taken care of, address the idle->full throttle hesitation issue with the low end needle valve adjustment outlined in the manual. The low end needle valve is inside the carb assembly and is adjusted with a screwdriver in the center of the throttle linkage.

Hope this helps.