A recent post about Keihin bike carbs on car engines pointed out the need to drill out the air jet and block it off. Opinion appeared to be that it was always needed. As my Skoda trials car is on those same carbs (from a Honda Fireblade) and I knew they had not been modified in that way I called Boggs who had originally set them up. The answer was that if the tune had indicated the need to do it they would have done it. So the answer is I think that if you are fitting a kit to specific carbs on a specific engine as here ( http://www.zen69595.zen.co.uk/carb.htm ) then do it but if not its best to get them tuned by a reputable and knowledgeable tuner (Bogg Bros etc) and let them decide if it needs done. Going that route if its not needed may well upset the running. Hope this helps the debate.