DOES FUEL GO OFF? YES!!
Fuel or Petrol does have a limited shelf life which means it only lasts so long after we purchase it. Just how long can depend on the type of fuel and how and where it is stored.
In general terms, it is not recommend to keep fuel for more than 30 days. This includes the fuel in your mower or outdoor power equipment. If it's going to sit for a prolonged period of time, drain the fuel (tank & carburettor) so when you want to use it again pour in fresh fuel and away you go.
Store fuels in clean containers with tight caps. Either steel or plastic fuel containers are fine but store them in a cool place. Areas with high temperature swings cause evaporation, oxidation and condensation. This includes your equipment as it has a fuel tank.
WHAT MAKES FUEL GO STALE?
Usually the first thing that happens is the lighter chemicals that are used to make what we call petrol evaporates away leaving behind a heavier less volatile product. Petrol in its ideal state vaporizes very readily to form a fuel/air mix to run our internal combustion engines. So when the more volatile components are gone the engine is hard to start, lacks power and doesn't run properly. The lack of power is more apparent in single cylinder, low compression engines as used in outdoor power equipment (mowers, chainsaws, trimmers, etc).
The second cause of stale fuel is oxidation - some of the Hydrocarbons in the fuel react with oxygen to produce new compounds. Using Oxidised fuel is bad for any engine. When oxidation becomes a problem the fuel gives off a sour odour. If you pour some fuel into a clear container you will see it has turned dark, you may also find small solid gum particles. The gum particles will clog the fuel filters, gum up fuel injectors and fine passage ways in carburettors. Engines left to sit for long periods with fuel in the carburettors when pulled apart are completely gummed up as the fuel has evaporated away leaving only the heavy gum deposits on the valve stems which will cause the valves to stick in the valve guide causing the push rods to bend. When this happens the damage is not covered by the engine manufactures warranty, it is the owner's responsibility to pay for the repairs.
Finally there is water contamination - Ethanol fuel is Hydrophilic, meaning it tends to draw moisture out of the air. So in theory it should become contaminated quicker. Water doesn't mix with fuel so it separates and sits in the bottom of the tank. When it gets drawn into the carburettor it can block the jets stopping the engine. Water from condensation in the fuel container is a major problem. Condensation happens from containers being cold and then hot and cold again. The water is heavier than fuel so it sits in the bottom of the container and when you empty the fuel the water runs into the fuel tank. Over time old jerry cans or fuel containers get small particles of dirt and sediment which mixes with the fuel and enters the fuel system of your machinery.