Using the time-independent power-flow equation, we have examined the mode coupling caused by intrinsic perturbation effects of step-index plastic clad silica fiber carrying more than 105 modes. Results show that the equilibrium mode distribution for this fiber is achieved at a length of approximately 550 m, which is longer than reported previously. While this coupling length is much longer than that of plastic optical fibers, it is shorter than that of all-glass fibers.