System version plist

Ive changed the system version plist in ifile to 5.1 to install iPhoto on my 5.0.1 jail broken 4s but now I'm having trouble installing certain applications from cydia. I couldn't install psx4all and now zephyr has uninstalled itself and won't let me reinstall it saying my iOS software needs to be 5.0 or greater. I've changed the version back to 5.0.1 and then back to 5.1 again but no luck.