That adjustment screw only affects the idle mixture, and since your problem is happening when the engine is running at speed, I doubt that's it.
There are two things I suggest you look at. First, check the rubber crankcase ventilation hose that feeds into the air cleaner and see if there is oil in it, and/or oil in the inner part of the air cleaner housing. If there is, the chances are you have a blocked positive crankcase ventilation valve or the tiny oil return hole from the ventilation compartment. I can tell you how to deal with those issues if there is oil in the hose.
Second, put a cup under the float bowl drain screw and then remove the screw, with the fuel turned on, and see if you get a steady stream of fuel, or alternatively you get a stream until it has emptied the float bowl, then it slows down. If it slows down, that would mean you have a partial blockage in the fuel hose, fuel filter, or fuel tap.
