You should hook up a hand held vacuum pump (MityVac), and pull vacuum on it, about 17 to 20 inches should open the valve fully.

I keep mine adjusted all the way out, I get backspin if I tighten it much at all. BTW, make sure you don't have your vacuum hose hooked up to the bottom port, that's for boost-assist, it needs a boost-referenced line hooked up to it, but you can just leave it open, or plug the port.