Those needles were never meant to be shimmed. I tried but could never get that big brass screw out. Was afraid I would bugger it up and from the look of the one in your picture I suspect you found it difficult. I also suspect that for shims you may be on your own and getting an assortment of different thickness washers at your local hardware store and doing trial and error. Undiablo has a H LTD model and I know he was able to remove the needles like you. Cant remember if he shimmed them but have not seen him here in a while. You could search his posts
You say no pods and stock exhaust. Have you played with the pilot air screw ? I'd suggest a 1/4 or 1/2 turn out to start. Stock setting is 2 turns out from lightly seated and from the factory that was a tad on the lean side. I have found that turning out beyond 3 turns out has no effect.
Fuel level in the float bowl will cause lean or rich running if lower or higher than specified (4mm) in manual . Getting fuel level (clear tube method) correct across the board was major for how my bike runs.
More turns out on the pilot screws is richer, not leaner. I use 2.5 turns out as a starting point. Agree that past 3 turns doesn't seem to do much. I also agree that removing that brass screw is a bitch; I tried a few times on the dozen or so CV34's I've restored and gave up because the slot was getting buggered.
Regarding shimming, be sure that when you lift the needle with a washer that there isn't any interference anywhere else. For example, does the needle hit that brass screw? Agree also that it shouldn't be necessary to shim the needle with a stock bike. My KZ750 actually runs a little rich if anything. Keep it stock and the bike will run fine if everything else is okay.