This is the  Main Air Jet  location on a Keihin FCR carburetor, as used on current (03-05) Yamaha and Honda offroad bikes.

The early Yam 250's are a bit different, but the MAJ hole location is in the same place.

I, also, put a pointer to the pilot air jet, just in case you wanted to ask what that "other one" was.

On some OEM (Original Equipment Manufacture), Keihin FCR carbs, it is necessary to drill out the stock  Main Air Jet  to use the Factory Pro HDJ emulsion tube's beneficial mods -

Remove the emulsion tube, the slide and the needle.

Drill through the Main Air Jet inlet with the supplied precision drill bit for your kit.
The MAJ entry orifice is in the "bellmouth" i.e. the "air inlet" end of the carb - like the top picture shows.

It's 46mm from the outside to the beginning of the emulsion tube hole.
If you have the emulsion tube out, you can just see the drill bit poke into that area - Stop! You've gone far enough!  :-)

Perfect. That's done!

Now - It's time to create a Factory Pro HDJ carb!

Using the supplied instructions with the Factory Pro HDJ kit, drill (if specified and supplied) and tap (if specified and required), follow the HDJ carb kit  instructions - and you will have a carb that will now perform better than any other "jet kit"!