well the tank being plastic or metal has nothing to do with if its clean inside or not
as you buy and use fuel, dirt can get into a tank, as can many other ways
quality of fuel also, can be a issue, as again, ethanol fuel can spoil FAST, its just not a great fuel if anything sits any amount of time
and you never know what is ina tank at station or how long its been there
so, IMO< I would drain fuel, start with known good fuel
the name of the game is, all about ruling things out
what yr is this ATV?
again, did this power loss suddenly happen or has it been loosing power for a while now?
and have you actually taken the carb apart and cleaned the whole thing, not just the bowel?
and if you did take whole carb apart, HOW did you clean it
as again this matters
just spraying with cabr cleaner doesn't clean a carb!
ethanol over time will and can build up a coating of varnish on inside of carb and its parts, blocking passages and such, and changing how the carb works
so, how one is cleaned matters and determines in if it was cleaned all the way or not, small things make a big difference inside a carb!
they sell carb rebuild kits, that can help things, by replacing some possible coated or bad parts<
and last is this a OEM carb, or an aftermarket carb??
is it an original carb, or did some one at some point change it out?