If these are definately replica dunstalls, have a look at either welding some of the outlet tubes up, or Weld a nut in the centre hole and make some discs that have varous holes or hole sizes in... Sounds like too little back pressure. I sourced some original dunstalls for mine.... Far better than the repo's... I have another (albeit) battered original set, and the outlet holes are smaller still.
But the problem itself won't be properly rectified with carburation alone. it'll run better but you will be constantly up and down the box, improving the back pressure will give the bike more torque.