Petrol fuel aging modes:

1. Evaporation. Depends on temperature and the availability of ventilation.
The fuel is a mixture of components that evaporate at different rate. A fuel depleted of volatile fractions may refuse to start burning in the engine. It may as well completely disappear or leave a small amount of oily substance at the bottom.

2. Polymerization. Depends on temperature, availability of oxygen and especially on the fuel quality. Light also helps if the tank, the tubing or something else is transparent.
Small molecules combine, sometimes all the way up to creating a glass-like substance (the evaporation helps to remove the still liquid components). It may clog to death the small openings in the carburettor / injector / pump / whatever and even require time-consuming cleanong or even replacement of the clogged parts.

3. Development of microorganisms. Depends on the bio-content (etanol, etc), but to some extent it happens even in completely crude oil derived substances. Also depends on the temperature, availability of water (condensate) and air. Most bacteria develop at the interface between the water droplets, fuel mass and air.

Bacteria leaves behind acids. Acids promote corrosion of the tank. Corrosion products (rust), combined with polymerization products acting as a glue and the bacteria colonies themselves make together a very good clogging agent.


2-stroke oil mixed in promotes both polymerization and bacteria development.


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How to start a long-stored vehicle:

Try syphoning some fuel from the bottom of the tank into a transparent container first.

- Clear, transparent, colorless or somewhat yellow (or whatever the original fuel color was), runny liquid of the expected amount = good.

May be mixed 1:1 with fresh fuel for better results.


- Opaque, muddy, brownish, viscous liquid, rust particles, fuel way less than what was left = bad. 

Fuel system cleaning required. Check for leaks or advanced corrosion in the tank.


2T oil also degrades, usually faster than the gasoline it is mixed in. If your bike is 2T, think about adding fresh oil. It is better to run the engine with twice the oil rather than with no real lubrication.

Be aware that 4-stroke motor oil degrades as well, by all of the above mechanisms. An oil change is highly advisable if the engine is stored for a year.