Just thinking...hypothetically speaking...what if you took the idle needle out and soaked it w/ o-ring, in oil? That would make the o-ring swell up some, huh? Maybe that would make it a tighter fit after it was put back together?
You'd clean all the oil off first and re-lube w/ fuel during re-assembly.

edit: Never having the need to dis-assemble one of my Evo carbs, I don't even know for sure there's an O-ring on the low speed needle but it feels like there is one when I turn it to adjust it...