I have not tried drilling the main nozzle or disc. However, since you have a flat spot at low throttle, I would look for a problem within the carb and not try correcting by making alterations. The alterations will, at best, mask the problem which will still be there.
You may have a clogged idle passage. I would also look at the little top hat shaped piece that sits between the body and the low speed needle's spring. That piece and the needle should not be allowed to be too worn. Too much clearance will allow in too much air for a proper low speed mixture. Too little clearance will not allow enough air in for the low speed air/fuel mixture.
Also look at the throttle disc where it meets the bore at the top of the disc for wear in both the bore and the disc. Being too worn will make it difficult to obtain a good idle.