Originally Posted by
Oomp
Looking at the picture you posted, it looks like part 'b' is a spring steel flap, and acts as a reed valve of sorts. I will bet that it only reduces airflow at idle, but as airflow increases, it bends downward into the channel below to allow more air thru.
I agree. A reed valve of sorts because it covers about half the carb opening. At lower engine speeds it's definitely blocking more of the airflow. But while the total volume of air is reduced at lower engine speeds this flap also raises the velocity of the air going past since the engine is still trying to pull the same amount of air in regardless of the flap. This increased velocity increases the signal on the Venturi. As engine speed increases the flap moves allowing a greater volume of air in while maintaining a more constant velocity across the rpm range. A more constant velocity should make fuel metering easier with less of a transition from low to high speed needles. I'm not a carb scientist so I could be wrong about the last part though. Definitely sounds like something you'd want to keep though, at least with this carb.