The mixture of ethanol and gasoline is only marginally stable. In ordinary usage the mixture is consumed within a matter of weeks. If the mixture is allowed to stand for 10 weeks or so, the ethanol will separate from the gasoline. As it is hydroscopic, it absorbs water from the damp air that it contacts within the fuel tank. Over time, this corrosive ethanol-water mixture, being more dense than gasoline, occupies the bottom of the tank, where it corrodes any metal with which it is in contact. Draining the tank will produce a nasty mixture of gasoline and "worms" of ethanol-water, filled with rust and debris. This stuff is what fouls carburettors; it doesn't combust well either.
I've been using StaBil through the past two winters, and the fuel in my bikes and my TR3 no longer show the effects of the watered-out ethanol separation. The sole purpose of StaBil is to extend the time that the ethanol and gasoline remain in a stable mixture. It's been around for several years, but I understand that the fomulation has changed as the concentration of ethanol has changed over time. It has no restorative capacity for carburettors, but appears to do what it is supposed to do very well. The longest time I've allowed a StaBil-ized tank to sit has been 4 months, so I can't really say how long it truely protects the fuel. It doesn't appear to effect the octane rating; StaBil-ized fuel seems to burn just fine.
