i am not sure for now how to solve it. The installation of Ubuntu 12.04 went without error messages and the bootloader install via yaboot went ok (no errors were reported). But if i reboot the system the powermac G4 hangs on a black screen were no further messages displayed. i dont know exactly what goes here wrong. I suppose that the kernel was not boot. I cannot change to virtual terminal via Ctrl Alt F1. Only boot via alternate install CD and rescue mode were i can chroot into the newly installed system works.
If anyone can point me what i can do this would be really appreciated.