Three-dimensional models, consisting of the flame kernel formation model, flame kernel development model and natural gas single step reaction model, are used to analyze the contribution of cyclic equivalence ratio variations to cyclic variations in the compressed natural gas (CNG) lean burn spark ignition engine. Computational results including the contributions of equivalence ratio cyclic variations to each combustion stage and effects of engine speed to the extent of combustion variations are discussed. It is concluded that the equivalence ratio variations affect mostly the main stage of combustion and hardly influence initial kernel development stage.