Now that I've belatedly looked at the manual, I notice that the main jet is not adjustable: only the idle mixture can be adjusted. You set the idle speed to 2,000 rpm with the engine warmed up (do not expect it to run slower than 2,000 rpm - it isn't supposed to, and it won't do it at all well. If you try to adjust it to run slower, you will probably foul up the whole tuning profile). To adjust the idle mixture, begin by setting the mixture screw at three turns anticlockwise from fully in (i.e. clockwise) - and be very gentle when you screw it all the way in, or you will ruin the mixture screw. Run the warm engine at idle and adjust the mixture screw until you achieve the highest possible idle speed. If it increases above 2,000 rpm, use the idle speed screw to slow it down to that speed before resuming adjustment of the idle mixture screw.
The air hose you mentioned is the crankcase breather. It connects the crankcase to a source of vacuum (the intake manifold). The end that pushes into the crankcase is supposed to be beveled, so there is nothing to worry about regarding that.
Have you checked the tappet clearances? They must be adjusted with the engine cold. Inlet should be .003-.005", and exhaust should be .005-.007".
Have you checked the quality of the spark? It should be blue, and it should spark evenly when you pull the starter cord.
