I have a lake here local that has a good population of grass carp. I haven't caught any on a fly but catch them (and turtles) fishing corn on the bottom. The biggest I have caught was around 17 1/2 pounds. Leeches seem like a good fly for them and check out:

http://www.carpanglersgroup.com/

Good carp info and a fly fishing section on their BB.

Steven