Try adjusting the throttle speed screw, left side of the carb. You will want to turn it counter clockwise. If that doesn't do it, try the cable adjuster at the top of the carb. Screw it in unitl there is just a perceptable amount of free play between the cable housing and cable adjuster.
If that isn't working, then you probably have an air leak in the intake, and that needs to be fixed asap before you burn up your engine.