If you take the air filter off and look down into the carb, there is a shiny "barrel" in the opening-- that is the slide. There should be small gap where air can pass through when the engine is idling. The throttle linkage pulls this slide open when you pull the throttle. Having the throttle screw turned all the way in doesn't sound right. Double-check that you are adjusting the idle screw; you can see the slide move as you turn the screw.