Reading Dataracers jetting guide, i see that drilling the holes on the carb slides out to 2.5mm can improve throttle response. Just wondering how it does this, i.e what are the holes for anyway, and why didn't honda make them that size in the first place?
I just like to know why im doing something before I do it!

As air rushes through the carb into the engine it creates negative pressure on the upper side of the carb slide diaphram, this lifts the carb slide. As you shut the throttle the negative pressure decreases and the slide lowers. the hole controls the speed the slide moves. If you have a slightly larger hole the slide will respond faster.
I guess that Honda use the 2mm hole as that suits a much wider set of uses, from fast riding to pottering round town.

ok, very well explained, thanks!!!!!!!!!!!