Backup with ROM manager doesn't work in some cases. So,in spite of ROM manager, Use "Nandroid Backup" using these steps:
1.Install NandRoid Backup App in your Device.
2.Ensure that your device is rooted by running Root Checker application
3.Turn off your phone and boot in Recovery Mode. For that press Home + Volume down + Power Key.
4.In the Recovery mode press Alt+B to run the Backup.
Either use Nandroid Backup or your custom recovery mode to backup your stock rom, these are safer methods.