Even ethanol free fuel goes off now days.
For some strange reason everyone is obsessed with ethanol.
Ethanol is the devils fluid so every fuel problem has to be caused by ethanol.
The actuality could not be further away.
Petrol used to be a distilled extract from oil, but the demand soon outstripped the supply so we worked out how to combine lighter fraction together and react lighter fraction with heavier fraction and then split up the heavier fractions to the point where you got noting from a barrel of crude other than petrol & tar .
Competition between the various brands force them to always use the cheapest processing to make what you put into your tank.
Modern petrol is no longer petrol.
It s a light fuel oil with some added volatiles so there is just enough vapours to start a cold engine.
LEave a bowl of fuel ( because it is not petrol ) out in the sun for a few minutes and it will change colour.
This is the volatiles compounds evaporating off , often called aromatics because you can smell them.
LEave it for longer and it will go very dark & start to smell like the swamp it came from .
Eventually it will turn almost black become really thick & sticky and will stay like that for months till it finally becomes tar.
40 years ago if you tipped a pile of petrol ( it was petrol back then ) into a container, like an old hub cap to wash the grease off the front wheel bearings before being repacked & replaced , if you went inside for a pee, the hubcap would be bone dry when you came back .
This should bring back memories for the over 50's on the list .
Modern cars & trucks are all computer controlled fuel injected engines and will start if there is 0.01 % ( by weight ) of volatiles in the fuel.
But your mower needs 2 % .
Old stale fuel can be used if you add some acetone or start your engine with some sort of starting fluid.
Once the engine is hot there is enough energy in the hot metal head & piston to evaporate some of the heavier fractions to provide enough vapour to start the engine .
Remember only gasses can burn so if the fuel can not vapourise then it will not burn .
And a carburettor does not vapourise the fuel, it atomises it in the hope that the increased surface area will allow enough volatiles to come out of solution to start the engine .