This is the center of the main jet stack, and it has small holes in the sides. The rating on this type is BE1-BE5. These ratings don't seem to have any rhyme or reason. For example, a BE4 does not necessarily let in more fuel than a BE2. Most people leave this stock and work with the main and air jets.
Jetting is something that takes trial and error. See our jetting blog for more info.
