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Fuel Vaporization: Causes and Cures

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If your classic coughs and sputters in the heat, it may be the fuel, not the engine.

Never had fuel vaporization? You’re lucky, but don’t be complacent because it can strike at any time. For those afflicted, crawling in traffic jams invariably leads to sitting on the hard shoulder with the hood up, stopping for gas can mean waiting half-an-hour for the engine bay to cool – and curing the problem can seem almost impossible.

With fuel supplies in turmoil and prices rising, refiners are introducing different blends of chemicals into fuel all the time. Refinery gasoline, which is made into petrol, boils at a range of 20-180 degrees C and, while you won’t get 20-degree boiling point petrol, other elements legally blended in current fuels can boil at temperatures as low as 40 degrees. That could boil off in your gas tank on a hot day: harmful to the environment, as gas vapor helps create damaging ozone in the lower atmosphere, as well as to your wallet.

More detrimental to your engine’s smooth running is fuel boiling in the fuel line to the carburetor, as that can bring your classic to an abrupt and embarrassing halt until the engine bay has cooled off enough to let it through.

Most modern cars operate a fuel circulation system, where cool fuel is constantly pumped from the tank and the excess returns to be cooled again, rather than sitting in the fuel line getting hotter and hotter until it’s needed – so gas refiners can get away with using blends that start to boil at lower temperatures: classics are not so lucky.

Fighting vaporization entails a four-pronged attack. First, find the best fuel for your car. Gasoline companies change the blend four times a year, with less volatile fuel for summer than winter, so if you’ve got winter fuel still in the tank, use it quick and fill up at a station that gets frequent deliveries. Shop around too, for not all gas companies use the same blends and you may find some fuels give less trouble than others.

Second, do everything you can to keep the fuel cool until it gets into the engine. That means using carburetor heat shields and manifold heat-soak blocks, re-routing fuel lines to cooler locations and wrapping them with reflective tape, moving an electric fuel pump to a cool location or fitting a heat-soak block to a mechanical fuel pump (or even abandoning it altogether in favor of a coolly located electric pump).

Also, make sure nothing’s restricting the flow or pressure: clean all filters thoroughly, replace any old flexible lines and check the pump is operating at full efficiency. Fitting a higher-pressure pump may help but if it’s too powerful for your float chamber needle valves you could be pouring fuel everywhere, with disastrous results. Ducting cool air from the grille to the carburetor float chambers will also help.

Third, make sure everything’s running efficiently. That means getting the carburetors rebuilt if they’re worn, keeping the ignition system in peak condition with its advance curve optimized to the engine, keeping combustion chambers carbon-free and making sure compressions are not reduced by worn bores or poorly seating valves.

Fourth, have the engine running as cool as possible. If the radiator is ten years old or more (less if you’re in a hard water area), get it recored – and, if the engine’s not had a recent rebuild, get the core plugs out and rod out the water passages. If the heater’s inefficient, have it recored too. Then the final trick – fit a lower temperature thermostat so the engine runs cooler.

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