Martin-A, no hard data, I'm afraid. I'm not a petroleum chemist, per se, (though I increasingly wish I had chosen that career.)
My guess is that the question may boil down (pun intended) to the level of impurities introduced in varying manufacturing processes, nature and origin of chemical feed-stock, subsequent storage conditions, the specification grade (purity) of the product, and the purpose for which it is intended.

The presence of air/oxygen, water, and sunlight may accelerate the degradation of minor components of petroleum distillates over time. The vast majority of components in petroleum distillates are, of course, chemically stable indefinitely. Added impurities tetraethyl-lead, ethanol, etc, etc, and impurities within the impurities may more easily give rise to by products leading to insoluble precipitates. (I'm taking a guess that that may be your problem.)

The presence of particulate metal (compounds) within a storage tank may also accelerate polymerization and precipitation of otherwise stable components.

BoFA
Ethanol is hygroscopic, I remember reading that powerboat owners in Florida had had problems when the ethanol content of petrol (gasoline for them) was increased and they didn't empty the tank. As ethanol corrodes plastic and rubber I'm not certain how this would affect storage in the ubiquitous red and green plastic cans used to store petrol these days. Here in France metal Jerry cans are still available but rust might be a problem for those (off topic I guess)

I have a carbed '93 mini that last summer had been off the road for a couple of years. It was _very_ reluctant to start, it would sort of fire but then run like a dog, coughing and spluttering and would hardly pull itself to the end of the drive. I suspected the petrol so topped the half full tank up with a couple of gallons of fresh petrol.

Then I noticed two of the plug leads were on the wrong way round, I think I was the victim of my brother in law's or friend's practical joke. When I sussed this the thing fired up and ran as well as it ever has (or as well as an A series engine with 75K on the clock could be expected to).

It ran brilliantly at least to the end of that tank of petrol, so I'm dubious about stories of stored modern petrol going 'off' very quickly.

Martin,

I've seen the float bowl and needle valves in a mini carb (su) get pretty gunked up when the fuel within evaporated over a couple of years.

You could take your carb off and clean it, but you might find the fresh new fuel will gradually dissolve and flush out any remaining gunk.

> I've still failed to track down any hard information on what goes wrong with petrol. My
> experience of an engine starting very easily and then running smoothly but then misfiring
> more and more as the throttle is progressively opened does not make sense to me.

Martin,

Do these symptoms not suggest a mechanical blockage/ gummage problem rather than a chemical problem? You're getting enough fuel through for idle/ low throttle running but as soon as you need more the mixture leans and you get the popping and banging?

"Petrol in a sealed metal can does not go off, provided it is purchased in the UK or any other more or less civilised place. It will be OK for years. About the only additive in fuel is the detergent which doesn't evaporate. In these lead-free days, octane number depends on the molecular make-up of the fuel, and all you need to do is stop evaporation. Even the smallest molecules in there (such as hexane) can't get through metal. Plastic, as used for fuel containers, is itself made up of long-chain hydrocarbons (usually high-density polyethylene) which have similarities with the short-chain petrol hydrocarbons, so small petrol molecules worm their way into the relatively large spaces between the plastic molecules and get out the other side. Losses can be several percent over long periods (months) but thick plastic helps reduce this. You can't beat metal with its compact atomic structure for storing petrol."

"Petrol is OK in a can with an airspace; a tiny amount of the more volatile fractions of the fuel will evaporate to fill the airspace, and that's the end of it. It becomes a 'closed system in equilibrium'. Molecules go back into the liquid phase at the same rate as they leave it and enter the gaseous phase. Even a three-litre airspace in a five-litre can will only contain a very small amount of fuel vapour, equivalent to one or two CCs of fuel depending on the temperature."

This link gives a good description of the problem and solutions:

http://www.swri.org/10light/fuelqual.htm

also, boat owners, before laying up for winter, always (well the wise ones do) top up their fuel tanks to the top, to stop condensation causing water to collect at the bottom of the tank.

In the link above, the description of what petrol is can be confirmed by putting a small quantity into a tall narrow jar, sealed well! and left. You will see it stratify into its different layers after a few months.

As for getting rid of old petrol, I put a pint per week into my wife's petrol Corsa where it burns off with the bulk of the unleaded.

I have no problem with the 'lighter' products of petrol 'worming' their way out of a can. The lightest material we know hydrogen is very difficult to store, with metal leaking like a sieve (ish).

A paper here: http://www.electrochemsci.org/papers/vol7/71110633.pdf discusses how hydrogen in a pipeline causes the very expensive steel used to construct the pipe to become brittle.

I feel that the 'lighter' petroleum products will have an easier time permeating through my red or green plastic can in the garage over winter.

I have found that stale petrol will cause lack of power and misfiring,because of the lack of the volatile components of the fuel,the engine will run as if the mixture is too weak.

The main volatile component in petrol is dissolved butane. It is added to improve startability especially in winter conditions, and also because it improves the Motor Octane Number of the fuel (which indicates its performance at higher revs, where as Research Octane Number measures performance at a slow idle). A wider introduction to the topic of gasoline blending for octane is here:

http://www.refiningonline.com/engelhardkb/crep/tcr4_29.htm

All those components will tend to alter chemically in a variety of ways as petrol is stored. Refineries like to avoid storing it for more than 6 months without re-blending (indeed, they usually prefer to blend prior to dispatch from stored components), even with tanks with a low air surface to volume ratio. Olefins are particularly reactive with air. The composition of petrol may vary significantly according to the processes used to make its components at the refinery - reforming atmospheric distilled naphthas produces branched alkanes, while catalytic cracking of heavier fractions produces more in the way of aromatics and other cyclic compounds, along with extra LPGs that may be alkylated to make iso-octane and other alkanes.

I had found it impossible to start my lawnmower engine after it had been standing a long time, after having almost pulled my arm out of it's socket! I stripped down the carb and found that the jet was partially blocked. After cleaning and replacing it I was still unable to start the mower. I then drained the tank and replaced with new fuel and, bingo, first time start.
I have a 60 year old BSA motorcycle which is ridden only infrequently and then only in fine weather (I know, Big girls blouse!). I have found this to start and run easily on low throttle setting with full choke. On the road it runs like the proverbial bear's arse at the higher revs in the higher gears and no choke and cuts out at traffic light stops on low throttle unless I put the choke back on.
The fuel used was very old and had been standing for a long time and this was the fuel I had transferred into my lawn mower.
I shall not be riding the bike until the better weather when I shall be using new fuel, but if it runs much better it will indicate that old stale fuel is not to be used.
Like previous comments I will get rid of my surplus old fuel by gradually introducing it into my modern car, with hopefully, no side effects.
By the way, I had found the bike carb all gummed up when the bike had been laid up for years previously.

"Like previous comments I will get rid of my surplus old fuel by gradually introducing it into my modern car, with hopefully, no side effects."

Somebody suggested I got rid of my stale petrol that way. I was not sure that a set of 12 burned exhaust valves would not be a side effect. The gamble did not seem worth the risk, so I burned it off bit by bit in an old cast iron barbecue tray.

Last week, I started my 2-stroke brush cutter using last year's fuel from a 5 litre can. It started readily but coughed and spluttered when I opened its throttle. I found that if I got it up to speed by partially opening the throttle under no load, then once it had warmed up fully, it would run without spluttering. This seems to confirm that a fraction that ignites easily had either evaporated or been oxidised. (Reminiscent of stories from the 1940's of starting a car on petrol and then switching over to paraffin once the engine was hot.)