Idle drop by 500 rpm?

A blown power valve will cause an extremely rich condition and the idle mixture screws will have no effect if it is blown.
No it won't. Tell me how. The throttle blades are closed at idle. There MUST be airflow across the boosters to pull fuel from the power circuit. The rich idle from a blown power valve is a myth that's been repeated over and over again until it's been believed and it's dead wrong.