AT 1 was the first 550 carb, you can adjust that one easier, on AT 7 you need to remove the brass plug.
I've had AT 1 on my JRed, it has worked flawless from day one, doesnt mather whatever weather I have.
you had AT 7 A since your 550 was from 15.
I visit and spoked with one mechanicer last week, they sell around 80 +/- 20 550 a year, neither AT 1 or 7 have been flawless.
Good thing is that Husky working to fix issues, but it aint good for us customers if we get an rotten melon.
That said, compared to the sale volume 550 has, there wasnt many projects I could grab from them.