We have studied the C 1s photoionization of acetylene using single- and multichannel scattering calculations including the core and selected shakeup channels. Our calculation found a shape resonance occurs in the 1 sigma(g)(-1) channel. However, this resonance is not the dominant feature in photoabsorption spectrum due to strong shake-up intensities at about the same position, in agreement with the high-resolution photoelectron spectra. The 1 sigma(g)(-1) and 1 sigma(u)(-1) channels have been identified as having different dynamics in the core ionization process. The calculated intensity ratio between these two channels agrees well with the recent measurement. Electron correlation in the target states is found to have a great impact on the computed cross sections and asymmetry parameters. In the coupled-channel scattering calculation, an insufficient treatment of target electron correlation leads to exaggerated channel coupling effects. A multi-electron resonant process via the doubly excited state (1 sigma(g)(-1)1 pi(u)(-1)1 pi(g)(+2)) was also found in the multichannel calculation. Results are also presented for ionization leading to some shake-up states.