I agree on the slide sticking... I had this problem on mine but it turned out the throttle cable wasn't moving smoothly and was sticking, i tried lubricating the cable a few times and finally gave up and bought a new cable for it and that fixed it once and for all.
Also check the area that the idle stop screw contacts on the slide. Sometimes people will adjust the idle without moving the slide up by pressing the throttle first and run the idle set screw into the slide and cause a dimple, the idle set screw catches on the bottom edge of that dimple and sticks the throttle partially open. If thats the case you can fix it, i usually take a dremel tool with a cutoff wheel and grind out the dimple and then take some sandpaper and smooth the end of the idle set screw if its no longer round on the end.
I would also check the jets and check the spark plug to see if its running to rich or lean.
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1984 Honda ATC200ES "Big Red"
1982 ATC200E "Hondie"
1988 TRX300FW "Project Quad" Still in progress....