If the main idle screw is cranked too far the blades will be into transition region and the curb idle screws won't work because you are no longer using them. This is a very common occurence. The valves being tight might be a reason you had to crank the fast idle out of the curb circuit too.

Also be very careful with the curb idle screws. Their seats are easily damaged and then they will never completely close off and might behave erratically (like my teenagers).

Kitz