Jerry has nailed the bit about too large running lean because of low air velocity in the venturi.
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https://www.thecarburetorshop.com/Carter_multiple_venturi.jpg[/img]
The link is a drawing of an experiment done in the Carter Carburetor School which demonstrates how fuel will flow based on different venturi take-off points. Note there are four OPEN glasses of water, with tubes leading to various points of a multiple venturi, and the relative heights of water that will flow due to the negative pressure from air velocity through the venturi. But the height of the columns will vary, proportionately, if the air velocity through the venturi changes.
Jon.
