WetLeather Motorcycle Rider Information

The reasonably factual story, I think:

Gasoline degrades for three main reasons (that I know of), with a fourth decisive factor that is not exactly degradation in the sense of the first three:

1. Exposure to UV light
2. Oxidation
3. Evaporation of volatile components and finally,
4. Adsorption of moisture

#1 is a concern when fuel is stored in translucent plastic fuel cans, dirt bikes with plastic tanks, and lawn equipment. Apparently the effects are pretty rapid, with measurable effects in 4 weeks. This is not much of an issue in a metal or opaque fuel tank.

#2 is a biggie. Of course there is the oxidation that occurs when the fuel is combusted, but in fact there is also a steady series of chemical reactions that break down the more volatile components of gasoline during storage.

Direct exposure to air is not necessary because oxygen goes into solution with the fuel. The oxygen in solution will be "used up" in chemical reactions with other components of the fuel.

Anti-oxidation is the primary area of activity for fuel stabilizers. Stabilizers chemically bond to some of the same molecules that oxygen affects, but when exposed to heat the bonds break allowing the fuel to burn. I am not sure but I imagine there must be some loss of energy associated with this process; nothing is free. Some of the energy of combustion must be used to break the bonds. It's probably insignificant for most practical purposes.

#3 As the volatile components evaporate or oxidize, the goo left over produces "varnish" which is what ultimately clogs the passages and jets in a carb, and the tiny orifices in fuel injectors. I am not sure if stabilizers have any significant effect in this area.

Keeping the tank full during storage is a practical counter- measure. The amount of surface area exposed to the atmosphere should be minimized. Remember that the tank is vented. Newer bikes will have evaporative controls, but some means of introducing air to the tank has to be provided or a vacuum lock would develop as the fuel is used.

During periods when the temperature goes high enough to cause evaporation, gasses will be produced which must be contained by the evaporative controls or vented to the atmosphere. When the temperature drops, a vacuum forms in the tank and either draws gasses back from the evap system or draws air into the tank. This "breathing" cycle introduces moisture and oxygen, and allows evaporative products to escape.

#4 involves "aDsorption" which is different than "aBsorption". In adsorption a substance in one phase (water vapor) is concentrated on the surface of another substance (gasoline) but the two do not form a solution. Water sinks to the bottom, being heavier than gasoline. There is also condensation going on, and that adds still more water. The point is that the surface area of the gasoline, if exposed to water vapor, will collect moisture. The practical way to deal with this is to keep the tank as full as possible during storage. BTW, the activated carbon in the evap system also works through adsorption.

In consideration of the above information, one can see that the use of a fuel stabilizer (Sta-Bil (tm) is not the only one available) is just as important for a FI vehicle as it is for a carbureted vehicle, with respect to prolonging the quality of the fuel.

Inside the carb, air is everywhere. Exposure to ambient pressure is an absolute requirement for the operation of a carb. That's one main reason why draining the float bowls is such a big deal; no where else is there the same surface area to liquid volume ratio combined with exposure to oxygen and moisture. The carb is where fuel will degrade the fastest. FI has no comparable device for mixing air and fuel even when the engine is not in operation.

However, FI vehicles do have a very sensitive device: the injector. For example, the new Honda FI injectors have 12 tiny orifices that are drilled with a laser beam. If gum forms there, it's going to plug those holes. Oxidation is the enemy, because it's at work in the fuel lines and in the injectors, due to the oxygen that is in solution with the fuel.

Based on what I've learned, here is my program: For a 1 month trip, I'd just fill the tank completely full and forget it. If it were a two month trip, then adding a stabilizer in conjunction with a full tank would be prudent. At 3 months and more, not using a stabilizer is asking for trouble, depending on the engine.

Long term storage of gasoline should be done in a sealed metal drum, such as those in which race gas is shipped. A stabilizer is still needed - remember the oxygen in solution. Gas stored this way is said to have a shelf life of 12 to 18 months, even longer if it's kept cool.

Of course, "shelf life" is a relative term when speaking of engine fuel. If you're burning the fuel in a 125GP engine, then the highest standards will apply; if you're burning the fuel in a lawn mower or generator, then if you can just get the engine started you're fine. A street motorcycle's engine will fall somewhere in between, with it's state of tune and sensitivity to detonation being the deciding factors.

This information was obtained from a variety of sources: SAE papers, Sta-Bil's web site, several gasoline FAQ's, research paper abstracts, etc. You could verify my conclusions with some work, or you could just take my word for it.

Glenn Minch