We report a detailed study of the electric transport and magnetic properties of the La_5/8-yNd_yCa_3/8MnO₃ manganite system. Substitution of La³⁺ by smaller Nd³⁺ ions reduces the mean ionic radius of the A-site ion. We have studied samples in the entire range between La-rich and Nd-rich compounds (0.1 < y < 0.625). Results of dc magnetization and resistivity show that doping destabilizes the ferromagnetic (FM) character of the pure La compound and triggers the formation of a phase-separated state at intermediate doping. We have also found evidence of a dynamical behaviour within the phase-separated state. A phase diagram is constructed, summarizing the effect of chemical substitution on the system.