This work presents a detailed analysis of the laboratory and astrophysical spectral data available for ²⁸SiC₂, ²⁹SiC₂, ³⁰SiC₂, Si¹³CC, and Si₂C. New data on the rotational lines of these species between 70 and 350 GHz have been obtained with high spectral resolution (195 kHz) with the IRAM 30 m telescope in the direction of the circumstellar envelope IRC +10216. Frequency measurements can reach an accuracy of 50 kHz for features observed with a good signal to noise ratio. From the observed astrophysical lines and the available laboratory data new rotational and centrifugal distortion constants have been derived for all the isotopologues of SiC₂, allowing us to predict their spectrum with an estimated accuracy better than 50 kHz below 500 GHz and around 50–100 kHz for the strong lines above 500 GHz. Improved rotational and centrifugal distortion constants have also been obtained for disilicon carbide, Si₂C. This work shows that observations of IRC +10216 taken with the IRAM 30 m telescope, with a spectral resolution of 195 kHz, can be used for any molecular species detected in this source to derive, or improve, its rotational constants. Hence, IRC +10216 in addition to be one the richest sources in molecular species in the sky, can also be used as a spectroscopy laboratory in the millimetre and submillimetre domains.