A reappraisal of the hot fuel handling problems originating in the carburetter has been made. Experiments have been made using carburetters with transparent sections that enabled the behaviour of the fuel in the float bowl to be filmed during various modes of engine operation. Under certain well-defined conditions of temperature, fuel pressure at the carburetter inlet and fuel composition, the occurrence of engine malfunctions coincided with the presence of gross amounts of fuel foam in the carburetter bowl. This interfered with the normal operation of the system and led to the expulsion of a large quantity of fuel through the carburetter vent and, because the vent was blocked by the foam, caused an overpressure to develop in the bowl. These events created an excessive enrichment of the mixture aspirated by the engine and caused severe driveability problems, which were very similar to those associated with the fuel starvation caused by vapour lock.