I can't find any reference to a model called Orion in the manuals section, if you have a model number I might be more successful.
The screw projecting vertically downward from the center of the float bowl is the main mixture adjustment. According to the Tecumseh 4 Cycle Overhead Valve Engines Manual, the preset adjustment for all models having that type of carburetor, is 1.5 turns from fully screwed in. Incidentally, with the screw fully in there should have been no fuel flow through the main jet.
