A carbon-cerium alloy liquid metal ion source (LMIS) with unintentional aluminum content is presented for generating focused ion beams of carbon ions, as well as ionized clusters with sizes of 2, 4, and 8 atoms. Emission-current-dependent measurements were carried out for the mass spectra and energy spread of all species, but focused on the carbon monomer ions and clusters. The full width at half maximum of the energy distribution was determined to be 6.5 eV for the monomer carbon ion and 14 eV for the light clusters at an emission current of 5 mu A. The source showed good beam performance when used with a mass-separated focused ion beam column. Applications to graphene structures, organic matter, and other carbon-containing materials are promising tasks for the new carbon-containing alloy LMIS.