Gas can sit in a lawn mower for around 5 to 6 months before it goes bad.
Although in some cases gasoline can start going bad in as little as 30 to 60 days depending on the conditions.
The polymers (varnish) will tint the color of the fuel while evaporation causes the lighter ignition vapors to disappear.
The result is a progressively darker, heavier, stickier and less volatile fuel.
When dealing with degraded fuel, do what you can to remove it from tanks and fuel systems.
Fuel varnishing is a gummy residue that clogs the carburetor.
It's caused by the deterioration of fuel that sits in the engine too long, and it happens in a surprisingly short period of time.
Depending on fuel quality and storage conditions, gasoline can deteriorate in as little as 30 days.
When storing your lawn mower the gas should be drained out or used up before the lawn mower is stored.
The most common cause of lawn mowers not starting after being stored is bad gas that was left in the gas tank and carburetor.
The gas eventually gums up the fuel jets and main jets in the carburetor which then means the carburetor needs cleaned or even replaced.
Overtime gasoline starts going bad and turns to varnish which is not good.
So always run the lawn mower out of gasoline before you store it longer than a month.
Gas contains volatile compounds that allow it to burn.
As these compounds evaporate, gas becomes less combustible and eventually turns into a gummy varnish-like substance.
When this happens, gasoline no longer smells like gasoline; it smells like pungent varnish.