THE NATURE OF PETROL Petrol is a mixture of many (even hundreds) of different hydrocarbons. Each component has its own boiling point which means that a fuel boils over a range of temperatures.

BP 100 Racing Fuel has a narrower boiling range than normal petrol and it best approximates the fuels produced earlier in the century.

THE EFFECT OF MODERN PETROL IN VINTAGE ENGINES
The effects range from poor vapourisation in the carburettor to excessive vapour formation in the fuel system. Each problem engine must be studied with a clear understanding of the likely symptoms. All of the following problems have been reported but vapour lock appears to be the most common. The possibilities are varied and may include the following cases.

Very early engines with wick or surface carburettors may suffer because the low boiling components may vapourise leaving the high boiling components to build up in the carburettor and hence lean off the mixture.

Some engines with simple carburettors that do not spray the fuel into small droplets may end up with unburnt high boiling fuel components going out with the exhaust. If the correct amount of fuel is added to the air stream but is not fully vapourised by the time combustion occurs then this is effectively a lean mixture. The air/fuel mixture in these circumstances could be likened to that of a cold engine running with the choke on. A lot of fuel being supplied but not much being burned.

In later engines the problem may move to excessive vapour formation in the fuel pump and fuel lines (vapour lock). This is caused by the high fuel system temperatures vapourising the low boiling components in modern petrol. This is the common form of vapour lock which results in lean mixtures.

There are several ways in which excess vapour formation in the float bowl can cause a rich mixture or flooding. Pressure build up caused by inadequate venting of the float bowl can force excess fuel from the jets. Secondly, with some fuel system designs, vapour bubbles formed in the float bowl as petrol ejects from the needle valve can cause foaming. The float then sinks in the low density foam which opens the needle valve and floods the engine.

Another complex problem is that of ice formation in the butterfly area of the carburettor. In some vehicles the carburetors have inadequate heat input, particularly if exposed to the air stream [motorcycles]. The temperature drop as petrol evaporates can freeze moisture in the air stream. The ice can restrict the flow of air and/or choke the engine causing power loss or stalling. This can occur below about 14°C with humid or foggy atmospheres.

BP 100 RACING FUEL WILL PREVENT THE ABOVE PROBLEMS
All of the above problems would normally be eliminated when using BP 100 Racing fuel because of the selected boiling range of the components. These fuels are designed for good vapourisation in the induction system while avoiding excess vapour formation in the fuel system [vapour lock]. The high octane will have no detrimental effect on the combustion process in these engines

THE EFFECT OF LEAN MIXTURES
To achieve complete and effective combustion, the fuel must be completely vapourised and the air/fuel mixture must be in the correct proportions. A lean mixture (insufficient fuel) burns slowly because of the wide spacing of the fuel molecules. Slow combustion leads to : 1. Overheating of the cooling system caused by the flame being there for a longer period. 2. Overheated exhaust valves, particularly if combustion continues after the valve opens.