In cases where a use permit shall be issued for the operation of a service station or repair garage, no junk yards or premises used for storage of junked motor vehicles or vehicles incapable of normal operation shall be permitted within the township. It shall be deemed prima facie evidence of violation of this chapter if more than six motor vehicles incapable of operation are located at any one time upon any premises not within a closed and roofed building excepting however, that a number not exceeding eight motor vehicles may be located upon any service station or repair garage premises, outside of a closed or roofed building.