Two things to look at: 1)You may have to re-set up the choke. At little bit goes a loooooong way at 5k'. Very little o2 to start with. 2) I think those carbs are air metered (as opposed to fuel) at the mix screw (the one you have at 2 out). Move them a half turn while idling and see if the RPM drops. The standard 2 turns out is baseline for sea level. I would guess you need to be around 1 or three (depending if the screw meters air or fuel). Try these in reverse order... the second one is easier...
