What do fuels stored for a long period of time in the carburetor likely contain?

Fuels that are stored for an extended period of time in a carburetor can develop problems due to oxidation and evaporation of their more volatile components. Over time, these processes lead to the formation of deposits that are often described as gum and varnish. This occurs particularly because fuel can break down and react with moisture and oxygen in the air, resulting in the creation of sticky residues that can clog passages and impair the functionality of the carburetor.

Gum and varnish buildup is detrimental to engine performance as it can impede fuel flow and disrupt the proper atomization of fuel, ultimately causing starting issues, rough idling, or stalling. It's important for any engine maintenance routine to include checking for and cleaning out these residues, especially if fuel has been left idle in the carburetor.

The presence of excess water or volatile gases is not primarily associated with long-term storage in the carburetor itself but rather affects the overall fuel quality in a different manner. High levels of octane are a measure of fuel's resistance to knocking and do not relate to the degradation of fuel that occurs over time in the carburetor.