High Ambient Temperature

[Ian McGowan]

"I am using super unleaded petrol but was unaware that petrol can boil?

Chris"

Carburettor percolation occurs when the fuel in the carburettor float bowl boils
either during or after a hot soak condition. An engine experiences a hot soak
condition when the engine load is reduced or the engine is shutdown after a
period of operation at high engine load allowing the heat of the engine to soak
back into the fuel system. Engine and fuel system component temperatures
are elevated during the hot soak. During the carburettor percolation event,
fuel is forced into the inlet manifold through the carburettor vent or jet system,
this may give rise to an over rich mixture which can lead to poor hot restarting
or persistent engine stalling, (27).
Carburettor foaming occurs when the fuel rapidly boils as it enters a hot
carburettor generating foam. The foam cannot support the weight of the fuel
float, which sinks allowing more fuel into the carburettor promoting
malfunction of the engine as excess fuel is forced through the metering and
vent systems causing the engine to run excessively rich. The resulting
potential engine malfunctions are poor hot restarting, hesitation and stumbling
under acceleration of the engine, (27).