The power valve is held closed by engine vacuum, and opens at a pre-set vacuum level to richen the fuel mixture in partial-throttle operation before the main jets become fully involved.
A bad power valve will cause extremely rich running if it's leaking, or a mid throttle stumble if it's not opening.
That adapter isn't doing you any favors, since you're dumping all the air and fuel against the flat surface between the primary and secondary openings on the 4BBL intake.
What problems are you having?