One reason for fouling plugs when everything else checks out is the incorrect installation of the main needle jet assembly. There could be a possibility of the spacer above the needle jet holder being installed improperly.
Look for the dribble of fuel coming out of the needle jet area in the throat of the carb with the motor idling. There should be no fuel flowing from this area if everything is installed correctly at idle, only when the slide moves upward with application of the throttle will fuel flow from this area.
Look for the dribble of fuel coming out of the needle jet area in the throat of the carb with the motor idling. There should be no fuel flowing from this area if everything is installed correctly at idle, only when the slide moves upward with application of the throttle will fuel flow from this area.