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Vapour Lock

by Takeo De Meter

We met a friendly guy who was driving his Minerva Series 1 at a meet in France last month on an unseasonably, nice warm day. Every time he had shut down his engine for a few moments, it refused to start again when still hot. He said „merde“ each time and I said „vapour lock“. I don‘t know the routing of Series 1 fuel lines off the top of my head, but I suspect that they run somewhere near the exhaust where some form of heat shield may be needed.

Comes the obvious question: what IS vapour lock ?

A: Vapor lock occurs when the vapor pressure of the fuel is higher than the surrounding environment. In older vehicles, with "sucking" fuel pumps close to the engine, the pressure in the fuel line to the tank is reduced by the sucking action of the fuel pump drawing fuel form a fuel tank nearly ten feet away, and when heated, the gasoline may actually boil creating a vapor of gasoline which the fuel pump cannot handle, thus the name "vapor lock"; i.e. it occurs when the fuel pump, which is designed to pump liquid, loses suction as it tries to pump fuel vapour.

Next question: what to do in case of vapour lock ?

A: Wait till the whole thing cools off so that the gaseous fuel in the fuel line condenses back to liquid.

Question # 3: What can be done to avoid vapour lock ?

In most modern vehicles, the fuel pump is submerged in the fuel tank and pumping (pushing) fresh, cool fuel all the time towards the engine, with a return line allowing excess fuel (and possible vapour) to return to the tank. No long suction line here. So, in a Series Land-Rover, the next best thing is to install an electric fuel pump as close as possible to the fuel tank and to make a provision for a return fuel line with a slight restriction (in the return line) at the T-junction you make near the carb, thus allowing for enough pressure to fill the carb bowl. In this manner, your fuel line will be pressurized for most of its length and the short suction section should stay cool enough to make vapour lock a thing of the past, even in your Series 1.

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