Keith Clark's CSS Parallax Inertial/Momentum Scrolling on Mobile
Not what you want to hear (assuming you put in some good hours reading through Keith Clark's guide) but iOS handles scrolling events differently than desktop. It's possible you could adjust what you have already, but if you really want this you're probably better off switching to a js tool with documented support for iOS parallax + momentum. Stellar can do this, for example - http://markdalgleish.com/projects/stellar.js/docs/