Thanks for your replies, everyone.
I have checked the float heights using the appropriate fuel level gauge a(clear tubettube that screws into bowlabowl drain screwascrew)s at the appropriate height. Actually, they are at the lowest of the height settings, so they are well below the lip of the carb body and float bowl. Actually, one float was set a tiny bit too high, so the fuel level was low causing the bike to starve for fuel at top gear and high speed.
Also, float needles and seats are new.
Baffling, isn’t it?
I just got something called Hylomar Universal Blue sealant. Gas resistant, used on aircraft, non-hardening. I’m going to try a thin coat on the gaskets.