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I bought some carb overhaul kits from Cruzinimage for my KZ rebuild and could not for the life of me tune the carbs to my liking. All cylinders were running lean with two not even firing.
Problem.....the O rings on the air adjustment screws are too small and do not form an effective air tight seal. Changed them out to the correct size and problem solved.

The Keyster kits have the same problem. The parts in those kits are very good otherwise.

I had to do the same thing on mine.

Good to know !

Everyone knows or should know that I have been pushing information regarding Ethanol blended gasoline and the effect it has on elastomers (rubber based components) All the carb kits available for the KZ1300 have Buna N Nitrile o-rings and those o-rings hate ethanol. You need to buy Viton 75 material based orings to work with the ethanol. They are more expensive than the Buna N o-rings but if $10.00 for a complete set of o-rings to rebuild 3 carbs is going to break the bank then sell the beast.
www.theoringstore.com will sell you a set of o-rings including shipping for around the $10 mark. This is what you need

[/Oring for the Neeedle Seat Body 7.5mm X 1.5mm $1.39 each
Oring for Idle Air Needle 3.0mm X 1.5mm $0.75 each
Oring for carb bowl drain screw is a part #007 since there was no metric equivalent that fit correctly. cost $1.05

This is really useful as I'm just in the process of restoring the carbs and have been trying to source the O rings for the carbs. However, I was also concerned about the ethanol in modern day fuels and what effect it will have on the rubber O rings. Found this site which indicates the resistance of different types of material used (to manufacture O rings) to resist ethanol. Doesn't mention Viton 75 though.

mykin.com/rubber-chemical-resistance-chart-3

The o rings for the idle circuit screws should (theoretically) never be in contact with fuel as they are right up the top of the screw on the outside of the thread.

Most other carbs have the o rings underneath the spring (right near the needle) so they are more likely to be in contact with fuel.

Still, good idea to use o rings that are ethanol resistant regardless of the location.