The crossed molecular beams method with 193 and 157 nm photoionization detection was used to study the competing reaction pathways resulting from collisions of ground state Y atoms with acetylene (C2H2). Three channels, corresponding to nonreactive decay of collision complexes, H-2 elimination, and H atom elimination, were studied as a function of collision energy (< E-coll>=6-25 kcal/mol). Production of YC2+H-2 and decay of long-lived complexes back to reactants were observed at all collision energies studied. Product translational energy distributions for the H-2 elimination channel demonstrate that a substantial fraction of excess energy available to the YC2+H-2 products is channeled into relative translational energy. Analogous H-2 elimination channels were studied in reactions of Zr and Nb with C2H2 at < E-coll>=6.0 kcal/mol. For these reactions, the H-2 elimination product translational energy distributions were found to peak near zero kinetic energy, in contrast to the behavior observed for the YC2+H-2 products. This suggests that a significant potential energy barrier exists in the exit channel of the YC2+H-2 elimination step, whereas no exit channel barrier exists in forming ZrC2+H-2 and NbC2+H-2. The reformation of Y + C2H2 reactants following decay of long-lived collision complexes was found to transfer 40%-50% of the initial relative translational energy into C2H2 internal excitation. The YC2H+H product channel was only observed to occur above a collision energy threshold of 21.5 +/- 2.0 kcal/mol. Since YC2H+H production is fully spin-allowed and involves simple Y-H bond fission in the intermediate HYC2H complex, it is unlikely that any significant potential energy barrier is present in excess of the reaction endoergicity. Additional studies of Y+C2D2 reactions confirm that the observed collision energy threshold for the H or D atom loss channel corresponds to the energetic threshold for reaction, allowing determination of D-0(Y-CCH)=110.2 +/- 2.0 kcal/mol.