when you looked for vacuum leaks,
did you start the bike and let it run for a minute, then spray degreaser or wd40 on all the intake joins,
eg carby to adapter, adapter to manifold, manifold to head
also check carby to fuel bowl too, (sometimes they can be warped)
and if the idle changes in any way when you are spraying the degreaser/wd40, then there is your leak.
i;m guessing here, but i reckon it may be at the OKO adapter to manifold join.
some aren't made too flat.
most of the adapters i've used were warped, and that's brand new ones.
take the adapter off,
put some 180 grit dry sandpaper onto a flat of piece of wood, glass, steel , then rub the flat mounting face of the adapter across the paper 5 - 10 times.
have a look at the mounting face and see if you can see scuff marks evenly across the face
the ones i have had, show up a lot of scuff marks on the 2 ends, where the bolts go through.
(it can warp them if the bolts are done up too tight too)
if it is, rub it back a bit more trying to flatten it out,
if you cant get it perfect then you may get away with running some thick gasket material inbetween the adapter and manifold.
so make up a gasket and put it back together and try it.
and another place to check for leaks is where the throttle cable enters the top of the carby.
if you dont have a rubber boot to seal it off, maybe try wrapping some insulation tape around where it goes in.
then start the bike and see if anything is different.
be sure to post up your results too.