A mixture compressing internal-combustion engine having a combustion chamber, an intake conduit for introducing a combustible mixture into the combustion chamber, a throttle valve for controlling the flow of the mixture in the intake conduit and apparatus for reducing knocking in the engine. The knock reducing apparatus includes a knocking sensor coupled to the combustion chamber, control means coupled to the sensor and an adjustable full-load stop positioned within the intake conduit adjacent the throttle valve. The stop is controlled by the control means to vary the full-load opening angle of the throttle by an amount corresponding to the output signal from the control means, a reduction in the full-load opening angle reducing knocking of the engine.