Storing fuel over long periods of inactivity can result in the fuel 'breaking down' making the fuel ineffective and potentially damaging to the engine.
A simple method to try and help prevent this situation is to add a fuel additive or 'stabilizer' like Star brite Ez-to-Store Ez-to-Start. This acts to control fuel break down and helps balance out the fuel chemistry while preventing undesirable chemical formations which can create resin deposits, gum and sludge, which can in-turn cause clogged fuel lines and injectors/valves, reduced flow through filters and corrosion of internal engine and fuel delivery components.
A tough protective coating on internal surfaces also helps to insure quick starts and maximum engine longevity, even after many months of storage at temperature extremes.
The Diesel and Petrol additives have different chemical components and application procedures so be sure to use the appropriate solution for your engine.