lean or rich.... ok, that wouldn't be my concern. IF those flaps don't come open, your in trouble. And, as you have discovered, TQ's and the such with a back flap cannot be set in the garage idling in neutral. Trust me, for years TQ's were the carb of my choice and all I ran. Tuning them to 340's, 360s, and even 318's in heavy trucks with tight converters, and no bog. Going lean on the primaries could be "offsetting" for the back flaps not opening. It runs better, but not the solution to your problem. I cannot believe a 625 Demon out of the box that was designed for a stock 350 Camaro is too fat for that 410 C.I. with headers and ported heads. I'm not there, but I think it's deeper than leaning out your primaries.
