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• Disconnect the chain and unscrew the white nut by turning clockwise. (Reverse thread).
• Remove  the handle assembly
• Remove the metal screw where the lever connects to the handle.  This is tight! Use impact driver and a vice if you can.
• Remove the white section and add silicon grease where it makes contact with the metal handle.
• Lubricate the metal shaft.
• Reassemble

Optionally, you could replace with new handle for about $15 on Amazon. Probably a better long term solution.

https://www.amazon.ca/Kohler-GP1034693-CP-Cimarron-Chrome-Finish/dp/B001GZVTBC

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The fill tube adapter broke. Found a 3d model online to replace broken part.

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https://www.printables.com/model/362087-kohler-toilet-fill-tube-adapter

Image Added

Unfortunately, this piece would sometimes pop out so I secured it with by drilling some holes and zip tiestying it.

References

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