getting the second venturi to open sooner would probably (not definitely) limit how far the first would open, I had the second seize up a while back, which prevented the first opening fully. To get it to open sooner you would probably have to make it open slower (bear with me) by increasing the distance of the link from the pivot on the second, giving you a more graduated pull starting sooner. I'm sure someone else could explain this better, or tell me I'm wrong (not unusual

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Marc