The two progression holes can bee seen if we peer straight down. We will be adding a third hole above the upper, smaller one.
The idle circuit gets a good vacuum signal in a very narrow range of low airflow, and is easy to adjust with the idle mixture screw. The main circuit gets its signal from a much greater amount of air flowing though the main venturi, and is tuned via jetting changes.
The progression circuit fills in the gray area that is just out of range of the idle adjustment, but before the main circuit kicks in. We are going to add a third progression hole to extend the range of the progression circuit.
