Big holes go back side ( toward engine) little hole lets a venruti effect though to let atmospheric pressure push fuel up the tube. Extra hole I'm saying add, goes on back side, where you are having the leanest reading on your plug. It helps to even out everything. Sometimes it's bottoms, sometimes top, etc. I have usually jetted for the top, and added the extra smaller hole down lower. That smaller tiny hole is about the right size to add in. That one already there is for Idle. If you jet it all good up top, and everything is good, but idle is lean, slightly widen the idle hole. don't add the extra hole if you are getting good mid and upper, just slightly widening the idle hole may do the trick. Before doing it, read up on the venturi effect and how your carb works. What I am saying will make more sense and you'll understand where you need to add it, or widen.