On the original Carter version of these carbs they used secondary air valves with different angles for earlier/later opening. If you look at the air valve you'll see it has an angled lip on one side. Changing the angle of the lip changes the opening point and I guess probably how fast it opens as well. To change the angle you should use a different valve rather than trying to bend it.
Another thing that some tuners have done is to drill the counter-weights to lighten them which also has the effect of opening the secondaries earlier.
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