Dump tube is the tube where the needle (bottom of the slide) goes into the fuel bowl, and into the main jet.
That is also where the main jet screws into. (On Mikunis it's actually removable)
It's also called the emulsion tube.
That needs to be drilled larger as well. I can't tell you what the factory ID is for that tube, but I can tell you you normally drill it out as big or slightly larger than the main. Ex. .122 dump and .120 main.
You can't replace that tube on a Keihin. So, go too big, carb is scrap.... Just like with gas jetting, higher elevations require less fuel.
So someone that runs .122 dump and .120 main at sea level could run .116 dump and .114 mains at high elevations (think Colorado, Wyoming, etc.)
The dump or emulsion tube's job is to, for lack of better terms, mistify the fuel charge into the cylinder. Mix air and the fuel for the incoming charge of fuel.
If you were watering plants, would you just turn the hose on full blast and soak one tiny area...or would you put the hose end on sprinkler or mist, so you cover more.
Similar...you want the fuel like a misty charge so it combusts more efficiently.
I know I'm not using the best terms or descriptions, maybe someone could help with the terms or wording, but that's the jist of it....