I suggest you post the Model, Type and Code for the engine, so we can access the Operator's Manual. At the moment I'm unclear on why you haven't mentioned the mixture screw. Also, IIRC, the idle speed is only different for the very old all-cast-iron engines, not for the I/C versions of the aluminium engines (I/C versions have a cast iron cylinder liner and a two stage air cleaner on what is otherwise a standard light duty aluminium engine).
