Method for controlling a fuel valve and/or an air valve supplying fuel or air respectively to a crank case scavenged internal combustion engine (1) comprising means for controlling said valve used for a supply system for combustible mixture to the engine, such as a carburettor (9) or a fuel injection system. The valve is controlled in consecutive periods of revolutions each having a period length (PL) of at least 10 revolutions. For each period a valve control sequence (N s /PL) is determined, providing a number (N s ) of valve shut-off positions (FCn) determining which particular revolutions in its corresponding period the valve is to be closed, where the number (N s ) of valve shut-off positions (FCn) corresponds to the amount of fuel or air respectively to be supplied to the engine (1) during the corresponding period.