I have one of the Weber(Edelbrok Performer) 4bbl carbs on my 454Mag. Having just completed an overhaul/cleaning of it, I am stumped on the procedure for adjusting the Idle Speed. The guide says to back off the idle speed screw until it's not touching the lever. Then measure from the air horn to the bottom of the pump S link and adjust the idle speed screw until you get the appropriate length. Where do they consider the "top of the air horn" to be? Where do you measure from?

I always adjust set the initial idle speed based on the transfer slot. I open the throttles just far enough so that you can see the edge of the slot from below (.060 inch)

Just put it on and fire the engine. Adjust the screw accordingly by the rpm. For the idle mixture, use a vacuum gauge or just make small adjustments to get it right.