This is what even relatively new carburetor float bowls can start to look like when left to sit with unstabilized fuel. Upon restarting that debris can loosen and clog jets, typically the pilot jet.

For better or worse, most of the gasoline you can buy at stations around the U.S. has been ?oxygenated? with some kind of additive since a series of amendments were made to the Clean Air Act in the 1990s. The idea is to help the gasoline burn more completely, and thus cut down on harmful emissions. The latest additive is ethanol, which — without getting into the political and environmental debates about its efficacy — is fine for use in fuel-injected vehicles that are run regularly and designed to use up to 10% ethanol (85% in flex-fuel vehicles).
On the other hand, ethanol-oxygenated fuel is not so great for any vehicles that sit between uses, and/or carbureted engines, like the one in your dirt bike or older motorcycle. Ethanol is alcohol, and alcohol is corrosive to certain parts in older fuel systems. Alcohol is also “hygroscopic” and likes water, so when water gets into fuel during a fill-up or from condensation, it can mix with the ethanol, creating a chemical combo that causes rust, corrosion, acids and sticky varnish that wreak havoc in fuel systems, especially carburetors. Ethanol can even cause rubber parts and fuel lines to dry out, harden and deteriorate prematurely.

Alternatives are few — unless you?re lucky enough to have a fu...