| Typically, this is the secondary air valve (AV) opening up too quickly. The AV opening is controlled by a spring (see tech article for adjustment tips) and the choke pull-off. If your choke pull-off is bad, then it won't do it's part, which is to keep the AV closed until vacuum drops.
A couple of other possibilities are: 1) secondary mixture ratio, which is a combination of your secondary rods diameter (larger = leaner) and hanger (lower letter = richer quicker); or 2) general ignition condition, such as timing, poor spark, etc., which won't optimize the air/fuel flow you're getting.
I'd start by checking your general ignition condition including timing, then the choke pull-off, adjusting the AV spring tension, and finally tuning the secondaries, in that order.
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82 Berlinetta, orig V-6 car. Rescued w/86 LG4/TH700R with all harnesses, sensors, ECM, etc. 2.73 open. Cat-back from '91 GTA, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LB9 w/ZZ3 cam, exhaust, paint, etc.).
57 Bel Air, my 1st car. Currently 396 .030 over, Weiand Action+, Edelbrock 1901 Q-Jet, Jacobs Omnipack, 1-3/4" headers, TH400 w/TCI Sat Night Special conv & shift kit, 3.08 10-bolt, AMSOIL syn lubes bumper-to-bumper. Best 15.1 @ 5800' Bandimere. Daily driver while Camaro was being put together. | 
