Microcanonical rate constants for the acetylene isomerization reaction have been computed using the direct cumulative reaction probability methodology of Seideman and Miller [J. Chem, Phys. 96, 4412 (1992); 97, 2499 (1992)] and Manthe and Miller [J. Chem. Phys. 99, 3411 (1993)]. Two- and three-degree-of-freedom calculations an reported using a normal mode Hamiltonian based on the geometry of the vinylidene intermediate. Due to the vinylidene well, numerous resonances are found in the isomerization rate. Little coupling is found between the CH2 rock reaction coordinate and the other normal modes, so that the resonances are readily assignable as normal mode progressions. Qualitatively similar results are obtained using two different potential energy surfaces, or different reduced dimensionality sets of coordinates.