I dug this up from links thru the FAQ's. In addition to the gum, or varnish issue, you get a reduction in octane rating and potentially water contamination. The gum will, well, gum up your carbs. But the fuel left in 'uncleaned' carbs has probably done more (than the stuff in your tank) as the volitles evaporate leaving the gum in all of the tiny holes and such. No real damage, but a pain to clean out. The lower the octane rating, the easier it is to burn. If it degrades to a sufficiently low level, it will burn with compression alone, no spark plug required. This isn't a good thing as the plugs will still fire, setting up two burn fronts. When the two fronts collide, you get knocking, or unwanted detonation within the cylinder. Old and/or water contaminated gas will not burn as cleanly, promote rust, etc. Even at $3.00/gallon, you're talknig about $10 in gas. Considering I'd rather chew off my arm than pull my carbs, I'd say it's worth a new tankfull. Put it in your mower if you just can't let it go.
Q: What is "stale" fuel, and should I use it?
A: "Stale" fuel is caused by improper storage, and usually smells sour. The
gasoline has been allowed to get warm, thus catalysing olefin decomposition
reactions, and perhaps also losing volatile material in unsealed containers.
Such fuel will tend to rapidly form gums, and will usually have a significant
reduction in octane rating. The fuel can be used by blending with twice the
volume of new gasoline, but the blended fuel should be used immediately,
otherwise teh old fuel will catalyse rapid decomposition of the new,
resulting in even larger quantities of stale fuel. Some stale fuels can drop
several octane numbers, so be generous with the dilution.
