Abstract: An engine operation control system comprising a vibration detector for detecting engine vibrations and producing an engine vibration signal, an engine speed detector for detecting engine speed and producing an engine speed signal, an engine combustion control device for controlling at least one factor which governs engine combustion, an engine roughness control device adapted for receiving the engine vibration signal to distinguish engine roughness and effect control for a time period on the engine combustion control device so that the engine roughness is suppressed, a control time changing device adapted for receiving the engine speed signal and determining the time period wherein the engine combustion control device is controlled in accordance with the engine speed so that the time period is decreased as the engine speed increases.