The simple answer is that it can seriously damage your engine. It should always be stored in a suitable container like a Jerry Can.
We never recommend using petrol that has been left unused for more than 30 days.
This is a common cause of calls to our consultants, people will often leave petrol in their machines (lawn mowers particularly) over the whole of the winter and then expect them to work fine.
What actually happens is that fuel can go through a number of processes, from evaporation, where the most combustable elements may evaporate and affect the efficiency of your engine to oxidisation.
You may well have had your old petrol mower for some time.OK. Motor fuels have changed significantly, over the last few years.
Modern motor fuel contains ethanol which deteriorates with age and contains alcohol which absorbs moisture from the atmosphere, causing the fuel to separate and go stale in as little as 30 days.
This can cause damage and starting issues. We recommend adding fuel stabilising additive to all fuel before filling garden machinery engines and never leave old, unstable fuel in your tank over the winter season.
Oxydisation is a much more serious problem, where Hydrocarbons present in the petrol react with oxygen in the air and change the petrol's fundamental composition. This can lead to deposits and impurities forming that can clog up the carburettor and other parts of the engine. This can cause expensive damage!
We strongly recommend you leave fuel in the engine for no longer than 30 days. preferably fewer.
So empty the tank over the winter season or whenever you are not going to use it for a long period and try empty out dregs regularly, filling up with fresh when you can.
Alternatively you could try using fuel preservative if you want your fuel to last over an extended period of inactivity.
So follow one of these three and your mower should survive inactivity with no problems...
Either: 1.Preserve & Stabilise 2. Use or 3. Empty.