When does petrol become stale petrol?

Capt. Clueless

Capt. Clueless

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I think we all know that trying to get an outboard (or anything for that matter) is extremely difficult if it has old petrol in it, but I wonder, how long it takes for petrol to become "stale petrol". I remember that you could put your mower away for a season with a full tank, and it would start the next virtually straight away.
I switched the fuel supply off and ran my Yammy F2.5 dry after last use, but it still has 1/2 a tank of fuel in it. Is a winter lay over with that fuel still ok or should I tip it out and use fresh next season? I hear so many say, "well it was working last year before I put it away".
 
You'll get lots of different views on this. However, I have 2 petrol mowers, a petrol strimmer, a petrol chainsaw and a petrol generator. After long periods of storage, they all start easily on the old petrol. So my conclusion is that replacing the petrol with "fresh" stuff is unnecessary.
 
I suspect the answer is "how long is a piece of string":-
How much air to fuel ratio in the tank?
How much fuel-surface exposed to the air
Air volume
etc

I once left a Flymo over winter, blighter would not start in April.
Old fella walking by said "Change the petrol"
Changed it, first pull - brrrrmmm.

Sorry to be of so little help, but you may or may not be lucky leaving it.
 
My 4 stroke mower is noticably harder to start, and runs less smoothly, if the petrol is more than about 6 months old and hasn't been treated but I guess a lot must depend on how it is stored.

Add a dose of "Fuel Set" or equivalent and you should be fine.
 
For a variety of reasons, I used my outboard only 2 - 3 times in 2016, so the fuel in it at the beginning of 2017 was probably 18 months old. I've only just finished the can, so I've been running quite happily on 2 year old fuel.

IIRC, it's important for petrol to be tightly sealed in an opaque container as the good bits evaporate quicker than the rest and light degrades some of the additives over time.

Personally, I'd leave the fuel in the engine and see what happens, but pick a time when it isn't a problem to drain the old stuff and give it some fresh to start it for the first time.
 
I have been using a 5l can of petrol that has been in the shed for 4 years with no issues this summer.
 
pvb said:
You'll get lots of different views on this. However, I have 2 petrol mowers, a petrol strimmer, a petrol chainsaw and a petrol generator. After long periods of storage, they all start easily on the old petrol. So my conclusion is that replacing the petrol with "fresh" stuff is unnecessary.
Click to expand...

Also echoes my experience with all my garden equipment. My petrol on the boat is 4 years old now and the outboard always starts first pull but that's in an air-tight jerrycan so I think that stuff will be good for another 20 or 30 years. :encouragement:

Outboard engines not starting after a period of storage is much more likely to be due to old petrol left in the carb float chamber which has now evaporated rather than anything wrong with the fuel left in the closed tank.

Richard
 
When I first had my BSA Bantam 2 stroke, it would never start after the winter break. I used to take it to a local bike man and it would return functioning again. I later found out that all he did was change the plug and drain and refill the fuel. When I found out, I did the same myself saving £50. So the answer to the OPs question is about three months.
Two strokes are very sensitive to fuel quality and crankcase air leaks.
I initially reused the fuel in my wife's Corsa and later in the 4 stroke petrol mower.
 
Capt. Clueless said:
I think we all know that trying to get an outboard (or anything for that matter) is extremely difficult if it has old petrol in it, but I wonder, how long it takes for petrol to become "stale petrol". I remember that you could put your mower away for a season with a full tank, and it would start the next virtually straight away.
I switched the fuel supply off and ran my Yammy F2.5 dry after last use, but it still has 1/2 a tank of fuel in it. Is a winter lay over with that fuel still ok or should I tip it out and use fresh next season? I hear so many say, "well it was working last year before I put it away".
Click to expand...

pvb said:
You'll get lots of different views on this. However, I have 2 petrol mowers, a petrol strimmer, a petrol chainsaw and a petrol generator. After long periods of storage, they all start easily on the old petrol. So my conclusion is that replacing the petrol with "fresh" stuff is unnecessary.
Click to expand...

On the other hand my Seagull outboard becomes unhappy with fuel more than about two years old and my Flymo won't start at all if offered last year's fuel. I do run the Seagull, in the normal course of events, on fuel left from the previous season but at the first hint of poor running I add some fresh.

It seems pretty obvious that some engines are more susceptible to the effects of old fuel than others but that aside scientific logic says that the conditions in which fuel stored is likely to be the major factor. Nigel Luther, at one time a regular contributor to these forums and who who you may remember runs fuel testing laboratories in the Baltic States, and I pretty much agreed that if fuel was to be stored it should be in tightly capped containers, in a cool dark place, with minimal contact with air.
To achieve this I transfer fuel I am keeping to tightly capped metal cans and only keep full cans, not part full ones, in a shed in a shady corner of the garden..
The overall aim is to prevent the loss of more volatile fractions, the oxidation of any component which might be susceptible to oxidation by prolonged contact with air and Uv degradation.

I always filter stored fuel before using, to which end I have an inline filter in a syphon tube, and for the man Evinrude outboard, at least, I mix it 50:50 with fresh.

The message that comes from outboard user forums in the USA seems to be that modern gasoline blends are more likely to give problems than the older blends and that the real trouble is the increasing ethanol content.

Maybe a fuel stabiliser will help. I've not tried one.

It matters not if a bit of garden machinery decides to have an off day, although if you have as many as pvb what you need is a gardener rather than fresh fuel. An outboard playing up on a dark , wet and windy night is a more serious situation
 
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With modern winters, my mower only gets a few months rest during the coldest time of year, whereas my outboard often has to go for much longer. I once had trouble with my then Suzuki 2-stroke which kept dying on me until I gave it fresh fuel, so I tend to believe that fresh is best. It isn't very difficult to syphon out the fuel from an outboard, burn off what remains and use the surplus from the can in one's car, unless it is diesel of course.
 
I use fuel stabiliser - made by Quicksilver I think but there are loads out there. It may well be snake oil but it's cheap enough to not care too much if that is the case.
 
Yep, always seemed to have trouble with petrol in lawnmower the following season, but I left my outboard (with a full tank) in the tropics for 15 months and it started OK after a couple of pulls.
 
As a matter of course, I put "fresh" 2 stroke on the boat and into the outboard every year. The "old" stuff gets brought home and stuffed in my strimmer and hedge cutter. I usually only mix up about 1/2 Gallon at a time. HOWEVER, this spring I found an old fuel can of 2-stroke in my shed. By old I mean >3years old. It was the "boat can" when I had a Mariner 4hp. I've had a Tohatsu since before strokers were "banned by the EU". (Brexiteers please note I do realise that this was agreed by the council of ministers and wasn't an arbitrary decision by the commissioners). My strimmer, hedge cutter and chainsaw have run perfectly all year on it. I just shake the can before filling up. This may have something to do with the fact that I never put them away with any fuel in the tank and run the carb dry each time I use it. I offer this as fuel for thought, the petrol I shall keep using.....
 
There are a couple of issues to bear in mind. Fuel stored in an outboard tank with the fuel tap and vent closed will last a while, but if you haven't drained the carb the 15ml or so of fuel left in the bowl will degrade very quickly.
Even if your engine starts and runs, I would be very wary. Modern 4 stroke engines need hardened valve seats to run modern unleaded fuels. The majority of modern small 4 stroke outboards not only don't have hardened valve seats, they dont have valve seats at all. Combine that with low octane fuel and you will encounter issues.
We are talking of a very small amount of fuel here...why take the chance of damaging your engine?
Use the stale fuel for cleaning your paint brushes
 
I had a plastic can of outboard fuel of about 2-3 years age, and when I opened the can and gave it a little sniff... It smelt like white spirit and was obviously no use...
 
I agree with Vic that proper storage is the key. I have a good quality 20l metal jerry can in which I store petrol for the lawnmower. 20l lasts about 4-5 years (small lawn). Never have any difficulty with old petrol at all. I live in a hot part of Europe too.

My father, on the other hand, has large lawn and is constantly having to buy new petrol - which he stores in crappy plastic jerry cans. He finds that even fuel that has been stored just one winter (in cold part of UK) is not good the next spring.

So I think it is worth buying decent jerry cans for petrol.
 
Ric said:
I agree with Vic that proper storage is the key. I have a good quality 20l metal jerry can in which I store petrol for the lawnmower. 20l lasts about 4-5 years (small lawn). Never have any difficulty with old petrol at all. I live in a hot part of Europe too.

My father, on the other hand, has large lawn and is constantly having to buy new petrol - which he stores in crappy plastic jerry cans. He finds that even fuel that has been stored just one winter (in cold part of UK) is not good the next spring.

So I think it is worth buying decent jerry cans for petrol.
Click to expand...

There could be an element of fact in that, used to keep petrol in old duckhams cans etc. lasted ages.
 
Plastic cans are fine, so long as they are properly sealed and kept out of strong sunlight.
Any vented tank or container will allow the fuel to degrade much faster.
All petrol contains various additives these days, I don't think it's wise to keep it longer than necessary.

I've certainly seen a few engines transformed from non-runner to runner by fresh fuel, although I usually at least clean the spark plug if someone's been trying to start a motor with duff fuel.
 
You will find that some small petrol engines suffer more than others, if you have a sensitive or problem motor it is better to buy SUPER unleaded petrol which will keep it's starting properties much longer than premium unleaded.
Always keep in a well sealed container out of sun light and away from heat. Any petrol you need to dispose of can be used in a petrol engine car as they are less prone to this problem.
 
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