High idles seem to be a more common problem these days. They were always around but I think now more common. I recently had to take the original carbs off my bike and mount my spare pair. I've used them before but never got them to work right. Now it seems the spare pair are going to be it because the originals need more work.
Check first that the problem isn't vacuum leaks around the intake rubbers.
On the new carbs I have now I notice the throttle plates don't close all the way around. They are a little crooked in the bore. I had fairly new new pieces in the old carbs. I was only able to get one of them out. So it was back to the dealers yesterday to get another shaft, throttle plate and O-ring. I already have a complete set of screws, 4, and I will reuse one plate, one shaft and one O-ring. I think I'll be able to put these on today. My complaint is the same as yours. A high idle after warm up.
Replacing #s 12, 13, 14 and 15. The spring on the right side, that is not numbered on this drawing, may or may not be on your carbs. If it is it sits on top of the slider to help it slide down a little faster.
The butterfly plates have beveled edges. When tilted in the carb bore they fit exactly. If you back the throttle speed screw off and hold the carbs up to a strong light, you may need to hold the slider up if there are springs on the slider, some carbs have this it is a popular addition to our older carbs, if no large springs on the sliders you can hold the carbs upside down, you should not be able to see any light around the edges of the butterfly, or throttle plate. If there is the plate is crooked and when the carb gets hot it hangs open more. See you tuned the bike with this in place and it seemed not a problem. But it fits less as it gets hotter. The plate sticks open more if it is not straight.
You may be able to loosen the screw and adjust the plates. This is sometimes not possible and the original screws are peened in. You need to file them down so they can be removed. What I do is I don't mess with them. I get new shafts and new plates and new screws and new O-rings. The new butterfly plates will have the correct identifying mark on them that insures you get them in the right orientation. I use Blue Locktight to put the little screws on because peening works better but causes so much trouble later.
The identifying mark? On each butterfly plate there is a very lightly tapped dot. Put there by some body in the Bing factory ages ago. This small dot goes on the outer edge on top. It is visible as the butterfly sits assembled in the carb. It may not of been there on the original carbs or if it was it is stamped so lightly that it is impossible to see now. I think it's worth checking that the ever so slight bevel I mentioned is properly aligned even after checking for the dot.
Again, look for vacuum leaks first and check for other stuff like timing or tuning the carbs, but if you are still plagued by a high idle after warm up that won't go away I suggest checking the carb butterfly plates.