The effects of magnetic perturbation on the quality of confinement in the sustained spheromak plasma experiment (SSPX) (Hooper E B et al1999 Nucl. Fusion39 863) are studied. The magnetic field is taken as a sum of an unperturbed axisymmetric magnetic field, B0, calculated from a magneto-hydrodynamic equilibrium, and a non-axisymmetric part, δB, representing the effects of kinking perturbation of the central plasma column. Poincaré maps show that flux surfaces are strongly affected by perturbations with δB/B0 ≳ 5%, with islands and stochastic regions covering a substantial part of plasma volume. The effective heat transport in the perturbed magnetic field, calculated by a Monte-Carlo procedure, is found to be consistent with the experimentally measured level of confinement.