Just my point of view but I would think that the way to go if you were going to tap the carb would be a standard M6 thread. The hole cast into the carb is very close to the tapping size of the M6 (5.1mm), so no drilling required. The tapping size for the 1/4" is 5.4mm so you may need to open the hole out to get the 1/4" tap to start. Standard 1/4" UNF is 28 TPI, although 20 and 32 TPI taps are available.
If you did need to drill the casting the drill is prone to snatch with the drill size being so close to the cast hole size, even if you "dulled" the end of the drill. Also you also have to align the drill with the casting hole which isn't easy by hand, especially if it does snatch. If you made a mess of the drilling you would then have to go down the thread repair routes
I would defiantly steer clear of having to drill the carb, especially if you don't need to as the metric size will tap without opening the hole up.
- 1/4 x 32" tap, what is the actual correct corresponding tap size - in metric?
There is no "corresponding" metric tap size. 1/4" x 32 UNEF is a standard size. UNEF threads are more likely used for adjustment screws where a fine pitch is required more than grip strength. In this case, I think Holley may have used the 32 tpi thread to force you to buy their jets down the road if you needed replacement jets. Brand loyalty so to speak.
There's always 2 ways to skin a cat.
Scotch gave us a Mig tip option with mods to turn the mig tip in to an orifice. Then Scotch found a Holley main jet option and posted the installation of the Holley jet.
In parallel with Scotch's research and development, and unaware of what Scotch was up to, I also struck off and found an air jet at
that turned out to be a Mikuni air jet. So both options are posted here.
Bottom line this mod development is all thanks to Scotch. His knowledge on this carb is invaluable and I bow to him. Scotch has reverse engineered a fix for one of the most troubling aspects of this fuel system on the KZ1300.
All works well, but in the back of my head I always have them sliding out some day inside the carbs - the fuel line/tube or other pressed in solutions. No matter how hard pressed in, kind of paranoia lol
A cumulative thank you for the other follow ups
1982 Kawasaki KZ1300 A4
Building an engine from random parts. This will eventually morph into building a bike from random parts - if it ever happens. Need a frame to begin with.