Is there anyway to update package installed with yaourt ?
yaourt -Syu seems to do the same as
pacman -Syu, which only care about package in official repository.
You can update your system including AUR packages with:
If you had read the manual pages:
You would have seen an entry like this:
In fact you'll find many other options in the man pages for things yaourt can do but pacman cannot. pacman was built keeping in mind the Arch Way.