I have been in the freelancing business for around a year or so (as a developer), so let me tell you what I think. Every now an then someone (on elance/odesk) comes up with an advertisement like yours wanting to build an app for Free, and share profits. There are 3 obvious pitfalls in this approach:
1) Can the developer trust you? How will he/she know for sure that you are going to pay bi-weekly profits and not just eat up his/her share? Its a different case when you know the developer personally but if you want to hire some one you don't know..its probably very difficult for someone to agree to this.
2)Can you trust the developer? If your idea is really a money making one, how will you ensure that the developer doesnt take the idea and make his/her own app?
3) Also, think about it like this. 100% of the work is done by the developer, virtually for free. And he only gets 60% of the profits (that may or may not come), and that too if he/she takes a leap of faith and decides to trust you. The developer account will be under your name, and if you are able to publish the app you will have full copy right. What will the developer get? Remember he/she has done 100% of the work for free!!
Therefore, i would strongly suggest that you change your business model and pay anyone you are interested in working with, upfront. That way you can ensure that you get a good quality product and will own 100% of the rights..which in turn means 100% of the profits. To get freelancers check out freelancing sites like Odesk/Elance.
Good Luck!