Further to 3G's point, here's some spec. info on the exact sizes and JIS tools which fit
Mikuni (and other JIS) carbs parts correctly......
Mikuni CV Carburetor Tools - Correct JIS metric sizes
Some may have purchased and used the "Mikuni CV Carburetor Special Tool" set, distributed here in the US by Sudco and their retailers. This tool set is made to Japanese Industrial Standards and is composed of correctly sized metric tools for cross-point and slot screwdrivers along with hex sockets & wrenches to fit Mikuni CV Carburetors. Carburetors and tools to service them both made to same design and mfg'g standards work best together. Stringently correct sized tools are essential for soft brass jets & other JIS fittings.
Vessel Tools and other Asian makers produce JIS tools and can readily supply the sizes of the Mikuni tool set. The following are the specific sizes that are needed to service Mikuni CV Carburetors-BS Series Mikuni supplied to Yamaha XS 650 OEM 1970-83
(sizes are metric)
Slot drive: JIS #4 & #6
JIS #4 (width 4mm x tip thickness 0.6mm) JIS #6 (width 6mm x tip thickness 0.85mm)
Just these two sizes only service all Mikuni CV carb jets & fittings. (Two more #6.5 and # 8 can have utility on other Mikuni Carburetors.) Note: no part of the shaft or drive is wider than the tip. (As most "spade shaped" US drivers).
Cross point drive: JIS # 2 (suggested additions #1 & #3)
Hex Sockets: 6mm,9mm Wrenches: 7, 8, 10, 12, 14 & 17 mm's
I have sourced all the drivers from Vessel Tools and found them to be excellent whether of 1/4" hex drive bit or various handled configurations and variety. Good information on them can be found on the site of their US Distributor and their outlets.
http://www.vesseltools.com/