Just a recent pic of two Mikuni carbs showing the before and after work. Someone handy or with experience can do this with a dremel. The carb bore is bored from the engine side and stops just short of breaking through to the bellmouth. This creates a double lip which can be smoothed out. Maybe some batches were better than others.
It gives the impression in the pic that you have made the bore larger but all that is done is unite the throat diameter with the bellmouth diameter. This gets rid of the turbulence causing step and creates a smooth radius and transition to the carb throat for better flow.
Next you radius the front edge of the lower slide to reduce slide flutter and improve flow.
Polish the bellmouth to reduce friction to the surfaces that face incoming air to increase air velocity and you get another 0.5 hp
.
Maybe it's worth it now FCRs cost a bajillion.
If you want to go nuts notice that the butterfly shaft is square and not round like many other butterfly carbs
. Then you can go really nuts and knife edge or "airplane wing" radius the butterfly itself.
Yep, Mikuni BDST38s, built down to a price.
However, I think the BDST 38 bellmouth has a better taper bore to it than the FCR, and a choke.
