petrol loses its potential over time and it becomes diluted by condensation forming on the sides of the tanks. your tank will have an atmospheric vent on it and over time continuous cycles of humidity temperature and pressure will cause water to condensate on the relatively cold tank walls run dwn and dilute the petrol making it less likely to ignite.
To remove the problem drain the fuel out of the tank and discard, in future decide whether to leave the tank empty or always keep it full to the top so that there are no exposed tank wall for the air to condense on to.
djh
April 2008
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