Most American pump gasoline contains ethanol, which works well in a vehicle driven regularly. The problems appear when fuel sits still for months in a machine put away for winter.
Ethanol pulls water out of the air
Ethanol is hygroscopic, meaning it absorbs moisture from humid air. A vented tank in an unheated garage cycles air through itself every time temperature changes.
Small amounts of water dissolve into the fuel without incident. Past a certain point the mixture separates, leaving a water-and-ethanol layer at the bottom of the tank.
That layer sits exactly where the fuel pickup draws from, and it is both corrosive to the tank and unable to burn properly when the engine is started.
The lighter fractions evaporate first
Gasoline is a blend of compounds with different volatilities, and the lightest ones are what make an engine start easily when cold.
Over months in a vented system those fractions escape, leaving heavier residue that ignites reluctantly. The fuel is not contaminated so much as no longer the mixture it was.
What remains oxidizes into gums and varnish that coat passages, which is why a machine that ran perfectly in October can crank without starting in April.
Carburetors suffer more than injection
Carburetors meter fuel through very small fixed passages, and a deposit measured in thousandths of an inch is enough to alter or block the mixture entirely.
Float bowls also hold fuel in an open vessel, so evaporation and residue formation happen directly in the component that is hardest to clean.
Fuel injection systems run at higher pressure in a closed loop, which resists evaporation, though injectors and pumps are not immune to degraded fuel.
Rubber and older materials react differently
Ethanol is a solvent, and fuel lines, gaskets and diaphragms made before ethanol blends became standard can swell, harden or break down in contact with it.
Replacement parts specified as ethanol-compatible resist this, which is a routine consideration when maintaining an older motorcycle or a piece of outdoor equipment.
Loose particles from a degrading line travel downstream into filters and jets, so a material failure in one place shows up as a running problem somewhere else.
Storage practice decides the outcome
Two approaches work, and they are opposites: fill the tank completely so there is little air to exchange moisture with, or drain the system so nothing is left to degrade.
A partially filled tank is the worst case, giving both a large air space and a body of fuel that will oxidize over the layup.
Stabilizer additives slow oxidation and help hold water in suspension, but they work only if added to fresh fuel and circulated through the whole system before shutdown.