The Causes and Control of Carburettor Foaming

Carburettor foaming and vapour lock are hot-fuel-handling malfunctions, both of which result in poor acceleration following a hot soak, but whereas vapour lock occurs in the fuel pump and is the result of evaporative processes during the hot soak, carburettor foaming results from the rapid boiling of fuel as it enters a hot carburettor. A foam is produced whenever the rate of bubble production exceeds the rate of bubble breakage; if sufficient foam is produced to fill the float chamber and block the air vent, the bowl pressure increases and causes the carburettor to deliver an excessively rich fuel mixture to the engine. The operating conditions under which carburettor foaming occurs, and the influences of both fuel volatility and mechanical design, have been examined. Foaming-related malfunctions can often be reduced or eliminated by modifying the carburettor or fuel handling system. Problems associated with carburettor foaming are likely to increase in the future if current trends in vehicle design, such as increasing aerodynamics, continue to raise underbonnet temperatures, and if steps are taken to eliminate fuel-pump vapour lock without due regard to the conditions causing carburettor foaming.