Check out this article on the tech and benefit of the flat slide type carburetor. For a long time I wondred if the factory VB series slide could be modified into a "flat" design, and after fooling around a while with it, this is what I came up with. That's a Delrin plastic insert, and of course the round part of the slide has been end-milled to produce flat surfaces.

Carburetors
VB series carburetors