A carburettor metering system comprises a fuel evaporator 113 consisting of porous parallel plates 114 with their lower portions immersed in fuel 121, fuel metering means 122 for supplying fuel to the evaporator 113, and a laminar flow air restrictor 128 comprising a series of parallel plates 127 separated by spacers and defining narrow gaps between the plates 127. Such an arrangement enables a substantially constant air/fuel mixture strength to be obtained over a wide range of air flow rates in single cylinder engines. Furthermore the supply of mixture to the engine by way of an exit tube 126 may be controlled by a valve member 132 coupled to the engine governor so that the rate of flow of mixture varies in dependence on the load, and additionally so as to change the system between two modes of carburettor operation, namely lean operation, which is provided up to about three quarters load, and rich operation in which additional fuel is supplied to the evaporator 113.