A helium plasma produced in a 82 kJ theta pinch discharge has been seeded with carbon ions by a carbon solid rod positioned on the axis. The first shock front of the discharge ablates carbon atoms from the solid surface which are ionized and interact with the plasma. Some shock front properties (namely thickness, compression velocity and its shape) were analyzed with an ICCD-camera system taking 2-dimensional photos of the plasma continuum emitted along the axis with a time duration of 100 ns. While the influence on the shape was not found, the temporal behaviour was drastically changed when the rod was added. The shock front occurs earlier and compresses faster. An explanation remains to be given. Furthermore, the influence of the carbon impurities on the plasma parameters has been investigated by measuring the electron temperature and density by means of plasma spectroscopy comparing these results with those obtained from discharges in helium without rod. We found a decrease in the electron density about 500 ns after the shocked plasma has started to interact with the rod surface. Simultaneously increasing continuum emission and decreasing intensity of the HeII P_a line indicates a regime of recombination. No influence was found on the electron temperature.