Does Fuel Degrade Over Time?
 

I always assumed that fuel degrades over time, having a half life time of some kind. I have a 16 gal fuel cell i tend to top up ever time It drops 4-5 gallons, so I can check my mileage and keep weight over the rear wheels.

If I top up ever few weeks, there would be some percentage of fuel in the tank from months ago. Wondering how potent that gas still is, and if it could be hindering my numbers.

depends on the blend. Ethanol is hygroscopic if the tank gets condensation inside the water will end up in the ethanol and a portion of the volume will be water.

When I lived in the bush the fuel delivered was a more stable blend because we would get one fuel barge before the river froze and the fuel had to be stable enough that it would be in good shape till spring and the next barge could get there.

As I understand it gasoline has a shelf life of 6 months before it starts to degrade enough to cause any problems, gasoline with eithonal has a shelf life of about 3 months.
As it ages the octane rating drops and the volital part of it dissipates as well, I've been told that Leaded gas, like air plane gas has a much longer shelf life of a year or more.

You might want to run your tank down to almost empty periodically to use up the "old" gas and assure that you have fresh stuff in there. Or use a preservative like Sta-Bil.

...that's a VERY interesting question, to which I can personally relate an answer!

...back in 1969, while flying in USN, we flew down to Acapulco, Mexico, to chase a tropical hurricane; the only problem was the only AVGAS (115-145 purple) they had there was left over from WWII and it had slimey green algae growing in it (and entrained water)! So, we had to "filter" every gallon pumped from their tanks through 'chamois' skins...and our EC-121 Super Connie held 8,758 gallons of gas!

...don't know how "long" that gas had been sitting, but from 1945 to 1969 is 24 years. The gas (after straining) was basically 99% usable, as the flight engineer said he only had to slightly back off on the ignition advance during METO power at take-off.

...by 1969 the Acapulco airport was handling only jets.

It does, but it's going slowly.
Whenever my bike stood still for months on end (even up to a full year), it had a full tank of "old" fuel to start with. I've never had any problem with the fuel nor with the carbs.

I work at a powersports shop in my free time. We run into more fuel related problems then you can imagine. To get rid of lead in fuel, they had to include other additives. Olefins settle out and degrade, forming a waxy varnish layer on the parts making content. Olefins, if I remember correctly, are what determine the knock rating of a fuel. I could be wrong, so a more credible source would be great.

I've also worked at a motorcycle shop and every spring people bring their motorcycles in that worked in the fall, half of them need nothing more then fresh fuel.

I wish it were only half by us. But we have ethanol.

I had a generator sitting in my garage for a year. I meant to exercise it regularly, but we used to lose power pretty often (snow in the winter; wind in the spring and fall) so I wasn't vigilant. This fall when we lost power, I realized it had been at least a year since I touched it. Luckily, I was good about putting Sta-bil in it, and it fired up without any problems.

That said, if I were using fuel, I'd try to drain the tank as much as possible. That makes the age of fuel cut off very sharply after the length of time between fillups.

Gasoline does slowly degrade but the trouble isn't with the gas itself, its what the gas does to components it touches while it sits.

The best example is when the gas isn't in a perfectly sealed container, portions of the gas slowly evaporate and others slowly degrade as the substances holding them in solution go away. The end result is a tough varnish.

Your carb usually isn't air tight and gets varnished rapidly when you leave a motor sit with old gas. As the gas ages it leaves more varnish after it evaporates, clogging your carb.

Even when gas is in a well sealed container it tends to varnish the surfaces it touches when it sits, just more slowly.

So the issue isn't with your old gas, it would be with you not running your engine for a period of a month or more often.

Cheers
Ryan

I guess my issue is that I have no baffling in the fuel cell so the more I drain it the more likely it is to starve the engine when decelerating, but I only run 91 ethanol free gas so it sounds like i have more time to play with it

The only advice I've ever read is to fill the tank as much as possible, to limit the extent of the evaporation (and reduce tank corrosion).

Yes the gas degrades with time. That is why I fill my 17 gallon tank after every 494 miles whether it needs it or not! :D

Or more often if my wife drives it and I don't feel like pushing!