lots of questions...
the carby should definately be set up with the choke off,
with the choke lever so it's making the plunger sit down into the carby.
all rubbers should make a decent seal, the throttle cable/carby rubber, the choke plunger to carby body etc
the breather vents should be open, allowing fuel to spill out if the bike is dropped etc.
the correct sized (26mm) plastic insulator with o'ring and gasket, can be bought off ebay for around $6-$8 delivered
or the one you match ported may still be able to be used, make up some paper gaskets using some 1.6mm thick gasket paper, one for each side of the insulator (incase the insulators' rubber o'ring is leaking)
i use 95 as a minimum octane, but usually BP 98, if i remember to buy it on the way.
i cant get 98 where i ride.
so clean the carby thoroughly with carby cleaner/throttle body cleaner, blow out all the orifices, passageways, jets, needle and seat etc etc with compressed air.
the fuel bowls sometimes warp on them too, see it you can face it off flat, with some 320 w&d paper on a hard block.
put the carby back together.
then set the carby fuel mix screw to 1.5 - 2 turns out.
set the idle speed screw so it opens the slide about 1.5mm above fully closed.
set the needle clip on the middle position.
start the bike and go from there.....
adjust mixture screw with bike warmish, slightly higher than normal idle, until best/smoothest rpm is found.
then drop the idle speed back until you have a decent idle.
then take the bike out and try it, the fuel mix screw may need a slight adjustment to get the best all over performance.
try adjusting the needle clip up and down, to find the best setting.
make sure you have a fuel filter, and a decent foam air filter fitted too.
let us know how you go, good luck