There is a special system that the 1971 455 HO carb has , its called the POE system . Its purpose is to add extra fuel at WOT . You may want to get the Quadra Jet book by Cliff Ruggles , he has a section on that system . Most Q-Jets do not have that system , it doesn't meter fuel as well as having fuel go through the venturies . You say your engine only runs rich in 1st gear , are you running a Ram Air hood and air cleaner ? Which body style , GTO , Formula , or Trans Am ? Each has a different system .
Check Fig 11
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