from
http://www.dansmc.com/gas.htm
Gas is neat stuff, but it has one bad habit (other than burning you up)... it rots, decomposes, turns into varnish and goos up your carburetors. A small amount of gas, like a pint, can go bad in as little as two weeks. A large amount, say 20 gallons, can stay good for a year or so.
This means that if you let the gas sit in a carburetor for any length of time you could be in for an expensive repair. Cleaning those carbs, on a four cylinder engine is a real pain.