For the flavour-singlet heavy quark system of charmonia, we compute the masses of the ground state mesons in four different channels: pseudo-scalar (ηc(1S)), vector (J/ψ(1S)), scalar (χs0 (1P)) and axial vector (χc1 (1P)), as well as the weak decay constants of the ηc(1S) and J/ψ(1S). The framework for this analysis is provided by a symmetry-preserving Schwinger- Dyson equation (SDEs) treatment of a vector x vector contact interaction (CI). The results found for the meson masses and the weak decay constants, for the spin-spin combinations studied, are in fairly good agreement with experimental data and earlier model calculations based upon Schwinger-Dyson and Bethe-Salpeter equations (BSEs) involving sophisticated interaction kernels.