We study in detail the flux properties of a radiofrequency (rf) outcoupled horizontally guided atom laser by following the scheme demonstrated by Guerin W et al (2006 Phys. Rev. Lett. 97 200402). Both the outcoupling spectrum (flux of the atom laser versus rf frequency of the outcoupler) and the flux limitations imposed on operating in the quasi- continuous regime are investigated. These aspects are studied using a quasi-one- dimensional model, whose predictions are shown to be in fair agreement with the experimental observations. This work allows us to identify the operating range of the guided atom laser and to confirm its promises with regard to studying quantum transport phenomena.