We studied polyyne formation by gas phase laser-induced breakdown in ethylene and acetylene gas flow using ns and fs lasers. The results show that acetylene is the most efficient target molecule for generating polyynes with high selectivity. Of the two lasers, the fs laser achieved higher selectivity for the production of hydrogen-capped polyynes. We also confirmed strong correlations between C2 radical and polyyne production, which have already been observed for larger hydrocarbon targets. In terms of the polyyne formation mechanism, we suggest decomposition of irradiated soot to be a possible pathway, in addition to carbon chain growth by binary collisions.