Two series of tests were made in 1918; one to determine whether the mixture giving best economy and that giving maximum power is a constant quality for all conditions of speed and power output; the other to ascertain what effect changes in the temperature of the fuel-intake system have on the quality of the mixture which gives the maximum power and that which gives best economy. The standard United States ambulance four-cylinder engine was used for these tests, its carbureter having a primary air passage, a primary fuel-jet, an auxiliary air passage with an air-valve and a secondary fuel-jet, the manifold being cast integrally with the cylinder block and a curved riser conducting the fuel mixture from the carbureter to it. The testing methods and fuel consumption measurements are described. The results of the first series are shown on curves having mixture quality as abscissas and torque and thermal efficiency as ordinates, demonstrating that the mixture quality giving greatest power is different from that giving best economy, and that the mixture giving maximum economy becomes somewhat leaner as the power output is increased. Similar curves from the second series of tests, considering changes of temperature of the air entering the carbureter, warrant the conclusions that over the temperature range investigated an increase in the carbureter intake temperature means an increase in thermal efficiency and that the mixture giving best economy at a high temperature is a leaner mixture than that giving maximum economy at a lower temperature. These conclusions are then amplified, a further conclusion being that from 10 to 50 per cent more gasoline than is necessary may be used in operating. The ideal carbureter would supply a mixture giving best efficiency and automatically supply the necessary additional fuel only when the operating conditions require this extra amount of fuel.