old gas?

Hate to sound like a really rank newbie, but what is it about old gas that makes it go bad (i.e. gumming up the carburator, fuel filter, etc). Is there something that settles out, or is there some kind of chemical reaction that leads to percipitate?
Thanks -Mark
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Where?
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wrote:

http://www.chevron.com/prodserv/fuels/bulletin/longterm_gasoline/detecting.shtml
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IT sits in a carburator and dries to Varnish...
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wrote:

Gasoline is a mixture of complex hydrocarbons. Some of these molecules are unstable and break down quickly. Some of the lower (lighter) hydrocarbons evaporate. What eventually remains are heavier hydrocarbons which are thicker and have higher boiling points. Gasoline starts to break down noticeably after just 2 weeks. A stabilizer can be added to *fresh* gasoline to prevent the breakdown, and this should be down to your gasoline-powered garden tools before putting them in to winter storage.
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Mark Modrall wrote:

It's oxidation which leads to semisolid materials often called gums and varnish that can plug small holes. It's more prevalent in modern fuel formulations because of additives that react with the gasoline.
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Mark,
I find that gasoline in any quantity still seems to work fine after being in a car tank/carburettor for years. I've started cars I own after being idle sometimes for months and sometimes even years, and they just start and run fine.
However, in small engines/carburettors such as in chain-saws, or outboard motors, then the fuel will go bad enough that the engine won't start until you replace the fuel. I suspect this is mostly due to evaporation of the light fractions of the fuel. It takes at least a couple of years for the fuel to get this bad in these small engines. I've never had bad fuel gum up a caburettor even under these extreme circumstances.
As for fretting over two week old fuel...... Well, virtually every car would have some fuel of that age in the tank. Ditto lawn-mowers.
After much experience along these lines then I replace the fuel once the engine won't start on it any more. And in doing it this way I've never had any bother once the new fuel is in the chainsaw etc, or had any trouble with the engine/carby subsequently.
Ross
wrote:

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Mark Modrall wrote:

Gasoline is referred to as an aliphatic hydrocarbon, or simply a mixture of difference hydrocarbon compounds. The molecules range from 7 - 11 carbon atoms per hyrdocarbon chain and include octane, heptane, decane, and nonane. This is the base gasoline which is then combined with various additives. The basic compounds that make up the gas do not evaporate at the same rate (non-azeotropic) which means over time some of the shorter chained more volatile hydrocarbon can simply vaporize. The volatility of the gasoline decreases which also decrease the solubility of all those wonderful additives. In turn the carborator will fill up with varnish (a medley of high molecular weight hydrocarbons or other additives) and the combustion process will show significant decreases in efficiency. Hence, under ideal conditions in a sealed, hermetic container gasoline will not go bad. The combination of evaporation, external contaminants, & moisture cause the gas to go bad.
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Mark Modrall wrote:

Must be the diet old people eat that gives them old gas. There is a reason they are known as "old farts" so I'm guessing it is the fiber to keep them regular.
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