Iphone width problems

So here is my website so far (it’s still being made): http://www.motodesignz.com/mx/

On a phone, you can’t see the left of the background, the website visually starts at the content box, I’m fine with that. However, you can see a bit of the right of the background. This problem occured when I put in the red navigation that overlaps the content box. The actual nav box has a larger width then the content box, so on a phone, the left of the nav is cut off, then you see the whole right part of the nav.

When there is no overlapping nav, you just see the content box with everything in it, you do not see the left and right background and I’m fine with that. But when there is the overlapping navigation, you can see the right of the background, I just want the phone user to see the width of the content box (they see only the width of the context box when there is no nav).

So my question is… How do I make an iphone user only see the width of my content box with the overlapping nav and not the width of the navigation?