Your outboard motor's fuel has four enemies while stored: evaporation, oxidation, condensation and contamination. Each of these is more likely to happen if you leave the vent on your fuel tank open. None of them is conducive to a smooth-running engine, and at least one can prevent the motor from operating at all.
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Like any liquid left exposed to air, gasoline and diesel fuel will evaporate. Both are highly volatile--very prone to give off vapours. Before your outboard can use any liquid fuel, it must be vaporised, the key process that happens in the carburettor before the fuel is delivered to the cylinders to power the engine. As fuel gives off vapours, it loses vital components. This is the reason you must start with fresh, new fuel every boating season, even if "old" fuel is treated with a fuel stabiliser. A tank of fuel that's lost its volatility through evaporation often can't start the outboard.
Oxidation, like evaporation, reduces the components of the fuel. Oxidation is more likely to occur after the most volatile parts of the fuel have evaporated. When fuel oxidises, the electrons of the oxygen atoms in the air combine with the fuel's atoms to create a form of varnish that, far from adding to the fuel's volatility, can leave varnish deposits on the walls of cylinders and the tips of spark plugs that inhibit clean, smooth operation of the motor.
You may think the fuel dock is watering its gas to make a higher profit, but the water in your fuel may not be its fault. The combination of heat, moisture and a mostly closed container is a sure recipe for condensation. Heat from the sun warms the air within the fuel tank. When the outside air cools as the sun goes down, the air within the tank may retain that heat longer than the air outside the tank. Just like the inside of an automobile's windshield on a cold day, moisture condenses out of the air on the inside of the cold glass that's located in a warmer environment. That condensation--inside the fuel tank with an open vent--waters your fuel.
When you leave the vent on your fuel tank open, it's an open invitation to bugs, leaves, bits of grass sprayed from a lawnmower or string trimmer, and anything else small enough to fit through the vent. While each of these has a place in the grand scheme of things, they won't work in an outboard. They might even plug up passageways--in the carburettor, for example--that are critical to the outboard's operation.
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