990 CFM? Never ever heard that one.
Stock TQ's are commonly refered to as 800 & 850 CFM units. The 340 had both on top. The largest Carter on top was a 630 AVS.
The 1250 CFM 6 packs in 4bbl. terms was a 780 rating.

Follow the above advice on the secondary air door. Tighten it up.