No, I'm not confused about the idle speed screw. It's got kind of a knurled knob on it so you can turn it with your fingers. And it has exactly the effect you would expect. The mixture screw is slightly recessed and is next to the carb air inlet. But I'm still not fully satisfied with the bike's tuning. It's all stock, with about 550 km on the odometer. I thought I had it pretty good, then this afternoon I took it out for a long ride -- 32 miles round trip. It ran great for the first 10 miles or so -- cruised fast and smooth at wide open throttle. Then it developed some extra vibration, made more of a coarse roaring sound (perhaps most of which was coming from the intake) and lost about 3 or 4 MPH. Got to the destination, ate dinner, and then started back home. Again, it ran great at first, then repeated the symptoms. When this happens, it seems like the bike will run well for quite a distance, but upon slowing down for an intersection or a turn, it will develop the vibration as I accelerate back to top speed, and then it won't settle down until the bike has been parked for a while. On the return trip, I stopped twice and turned the adjustment screw in (clockwise) about 1/8 turn each time, to no effect.
I'd really like to get it to run fast and smooth consistently. Any suggestions?