First, you have several threads to re-tap to make that engine usable: the one for the auto choke cover is one of them.
Second, it is not essential to remove the long pickup tube. You can spray the carb cleaner across the mesh, and clean it that way.
For the mixture screw, begin by checking how many turns it is screwed anticlockwise, from fully in. Be gentle doing this: you will ruin it if you screw it firmly against its stop. Just turn the screw clockwise gently, counting turns, until it bottoms lightly. Report how far it turned. Then unscrew it completely. Inspect the tip as described in the manual, and clean the screw with carb cleaner. Inspect the metering holes in the bottom of the hole the screw comes out of, without touching them, then blow carb cleaner through each hole in turn, noting where the liquid comes out. Then put the mixture screw back in, adjusting it to the standard setting given in the manual. You'll adjust it for best running when you actually start the engine. Then check the bottom of the carburetor, and the matching part of the top of the fuel tank, for flatness as shown in the manual. Note the specification for flatness: it is quite critical.
