carb issue for sure,
check the tube between the carbs is not cracked and seated properly.
If that is it, are the slides in the correct carb? swapped side to side is BAD
have you changed the pilot to a 27.5? if not, do that, and while you have the carbs off put them in a can of carb dip overnight.
Pull them and clean every orifice with brake cleaner and blow out everything with compressed air.
I would run torch tip cleaners through main and pilot jets, just as an SOP thing for when I go through carbs...
Once assembled and back on the bike
LIGHTLY seat the air screws, then turn them out 1.5 turns as a baseline.
eyeball the "sync" before you assemble the rest of the intake, tank, fuel lines
and again at this point I would remove the petcock and flush out the tank, clean the screen filter on the petcock and put it back together....
new fuel lines and filter(s) drop tank on, prime fuel system and fire it up to see if it is fixed.... It idles and revs out good without load? Final assemble and take it for a putt....