Did you manage it? For me I've only ever had to click them both and that was that. Never had to keep clicked and move the mouse around like for radiators or somesuch.
On this subject... anyone know what the third fuel cock is for?
Also, Walshy, the Crosscountry mission is particularly hard for takeoff because of cross-winds. You might want to set up your own mission or fly online
Good luck, and let us know if you have any problems!