1; yes just drop the zero for the main jet size, standard jets for the 28 are 140's with 75/80 powerjets, they aren't 55's thats for sure if it ran nicely ever.
2; 190/192 is a couple of sizes too rich on mine in summer (HRC card, Tyga pipes, drilled airbox, the UK) draw your own conclusions.
3; the best way to change the needles is to take the whole needle carrier out, yes it does appear you have to mess up the carb balance and no I don't have a good way to get it spot on again to be totally honest.
4; Don't know where the powerjets were located is the easy answer, I didn't fit the jet kit originally and the jetting was a mess when I got it, the '28 does run power jets though.
5; There doesn't appear to be a different x piece on mine, it looks like a std black one and it's "open end" is still blocked with the cap, there are 2 more (presumably) spare caps it the jet kit as well. It is running the HRC Y piece, you can mess about with X and Y pieces and their jets for the rest of eternity if you want.

Really as a base all your after doing is ditching the powerjets (and hence running a lot bigger mains) and fitting the adjustable needles.
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