Taken off the internet
Unleaded petrol can become stale after two to four weeks (especially in the summer). As it becomes stale, it turns into a varnish that coats the parts of the fuel system, especially inside the carburettor and the jets in particular. This varnish is difficult to remove and normally requires the carburettor to be disassembled and cleaned with a solvent in an ultrasonic cleaner. Even if fuel is drained before storage, enough petrol can remain in the carburettor to cause problems. It also loses it’s combustion values and will make the machine very difficult to start.
This varnish like substance can also coat the spark plug, making it difficult to create a spark. To help avoid this problem, it is advisable to use a fuel stabiliser in your petrol, this can keep your unleaded petrol fresh for up to 2 years. It also helps clean the fuel system and eliminates the need for end-of-season fuel draining. You can either add this stabiliser to the fuel as you place it in the fuel tank, or you can add it in the petrol can and mix the correct ratio petrol and fuel stabiliser in the container.