adb.exe android gives me error with su

I rooted my device (lenovo 2107a) and then updated new custom firmware with below instructions: - Downloaded new firmware into sdcard/clockworkmod - booted to CWM - selected "backup and restore" -> "advanced restore from internal sdcard" - and chose my custom ROM - Then i selected "system restore" So i saw an error that said cannot restore and restore goes to break. now when boot my device i see a screen that wants to select country code and after select device will reboot again and i see same screen. so i decided to restore my old backup, but even old backup restoring gives me the error that cannot restore. i find the problem, when i open shell with this command: adb.exe shell $su su segmentation fault the problem is that all of data in /system is read-only and i cannot even root with su. i tried everything like re-rooting device, but still i cannot shell as root. mount cannot be run (operation not permitted) adb.exe remount (cannot be execute) fixsu.sh cannot be executed because has not access to /system (everything is read-only) psneuter gives (failed to set prot mask (inappropriate ioctl for device)) is there any way to delete /system and re-upload that or something that can solve my problem?