You need to set the idle at 450 RPM before you set the mixture screw for highest RPM, then back the idle screw back down to 450 RPM and readjust the mixture screw to highest RPM. Repeat this until the idle stops climbing past 450 RPM when you tweak the mixture screw.
With the RPM at 800-1000 RPM, the throttle plate is open too far, uncovering the holes in the throat of the carburetor allowing extra fuel in. This will void any adjustments made with the mixture screw.
