detach the throttle cable from the carb linkage/armature plate pivot point...does the cable move in and out freely when you twist the grip?..if not, try to determine why..if so, try to move the the armature plate linkage and move the plate..does this move freely now or is it binding at this "rough spot"?...if it's binding, you will have to remove the armature plate and try to determine the cause, it may be that lightly greasing the armature plate bearing restores freedom of movement..anyway, that's 2 possible causes for what you are describing