The manufacturers seem to recommend that you make the idle richer than it would be to give the best idle quality, because of that problem. I suggest you try readjusting the idle mixture (low speed mixture screw). Recheck that what you have gives the best-quality, fastest idle, then turn the screw anticlockwise up to an eighth of a turn, speeding up the idle when you do it so it runs fairly smoothly (though it will be four-stroking). The clutch may not disengage, because the speed will have to be a bit higher than before to keep it running stably with the rich mixture. Then run it flat out and adjust the high speed mixture for the fastest speed, then turn it anticlockwise until it just starts to four-stroke. That makes it a bit rich on the slow-down when you release the throttle, and gives it some chance of not stalling.
Welcome to the world of two stroke brushcutters.
