Keihin CV34 needle retaining nut

Hi guys.
I’m trying to remove the needle retaining nut in the carburetor slide, so that I can I install a shim under the needles. The nuts appear to be almost frozen in position and I am in fear of screwing up the flat head screw driver slot if I try and over torque it. Has anybody had similar issues and/or advice as to how I can remove them. My bike runs way too lean in the upper mid range and raising the needles seems to be the logical fix. Any and all help would be appreciated. It’s a 1981 KZ750 Ltd by the way.

I've noticed the same thing before; that screw is really hard to remove sometimes (everytime?)

Maybe some heat from a soldering iron?  Just don't touch the diaphragm or it will be toast.

Ok I’ll give that a shot and locally heat the slide with a soldering iron to see if it expands enough to loosen the nut. Failing that I was considering an impact wrench. My fear with that though would be destroying the slide or the nut or both.
thanks for the idea by the way, much appreciated!