How "old is old" for petrol?

Robert Wilson

Robert Wilson

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Two outboards: Mercury 2str 4HP and Honda 4str 2.3HP
Both have to run on choke (greater/lesser extent) until hot, but will die if choke pussed-in too soon, even after three or four minutes.
Mercury mixture 50:1 possibly put slightly too much oil in mix. Honda just serviced, started/worked fine until yesterday.
Petrol bought last September/October - Could this be "old" now and causing the problem

I once had a 2str Flymo grass-cutter (petrol prob'ly year old) that would not start - put fresh petrol in and started first time.

Surely 6 months isn't too old for petrol? Any other suggestions?
 
Each Spring I dismantle the carbs on my Mercury 2str 5HP outboard and my Honda 2.0i genny and clean and blow the jets after they've sat idle for six months over winter. The "old" petrol sitting in them does go sticky and does gum up the jets (the slow running ones in particular). At the end of the summer season I put all the remaining petrol from the outboard, genny and tanks into the little Suzuki jeep we run around in. The oil makes it smoke for a bit but I've learned not to use petrol that's been lying around over winter in the outboard and genny.
 
Depending on the conditions of storage petrol changes properties significantly in as little as 30 days, the main effect being an increase in density due to the loss of the more volatile elements, plus some oxidation of the fuel itself. This would tend to make starting more difficult. Octane ratings decline only slightly, which would only be an issue in high compression engines (which would not include your outboards). The sluggish warm-up you describe could also be expected to be worse in cool, damp conditions, which is what I daresay you're experiencing now.

Fuel companies commonly change the formulation of fuel between seasons, adding more volatile elements during the colder months. Thus a nine month old 'summer' fuel might conceivably be better than a three month old 'winter' fuel.
 
My strimmer started and ran perfectly well on petrol/oil mix that was seven years old. The petrol in my outboard is at least two years old, runs perfectly. So long as evaporation is prevented by keeping the lid closed the composition is not going to change.
 
It's a never-ending discussion. I haven't had any problems with old fuel. Others claim it to be a regular issue.
Having said that for the past 20 years my garages (particularly whilst we've lived in the Lakes) have been cool all year round, so mower, leaf blower, hedge trimmer, chain saw, small outboards have been kept in dark, cool conditions.
Having said that the outboards on my boats have been out in the open, but used every couple of weeks at the very least.
 
I don't appear to have any problems either, I just top-up the old with new when required, plus you do have to maintain your engine (ACF 50 best product available) (although I think 'Corrosion Block' is the same chemical composition.) To many people have said "while it's still running leave it alone" .... & that is OK (for a while) ..... but when it eventually gives you problems (and it will) they will be 10x more of an issue which could of been avoided with basic regular-ish maintenance.
& if I can do it ... anyone can.
 
I've been successfully using 2-1/2 year old 100:1 in a mariner 5 2/s. As Vyv says, the crucial thing is to remember to close the tank breather after use, especially if it (like mine) basks in a sunshine-bathed RIB all year round (Clyde? I wish...).
Possibly there's a hint in the original 100:1 ratio, it's probably a lot less than that now, but not enough to upset the mixture balance.
I used to 'demote' old O/B fuel to the garden tools or car, but the Mariner likes it so much there's no need.
 
As has been said the secret is in the way the fuel is stored

I store mixed two stroke fuel from one year to the next observing the following


Full cans only to minimise air space and oxidation of certain components of the fuel.

Metal cans to exclude light and avoid diffusion of oxygen through the walls of plastic cans

Tightly capped to prevent loss of lighter fractions (and ingress of air)

Always filter before reuse. Once I did not and ended up stripping and cleaning a carb in the dark while on one of the buoys outside Cowes.

Mix 50:50 with fresh before use.​


I agree the Flymo does not like stale fuel ........ nor does the Seagull ... I never try the Evinrude on fuel which has not been stored as above and mixed with fresh

As alcohol content is increased expect increasing difficulty with stored fuel due to it being hygroscopic. Use of a fuel stabiliser may become necessary. "STA-BIL" is popular with outboard users in the US
 
As stated above "Old" fuel is anything over 30 days. If not kept in perfect conditions it will start to "Varnish".....which is... go into a jelly like substance that will give you a very big pain in your wallet, unless your able to completely strip the fuel system, clean it and put it back together again, which i dont think you can do because if you did you wouldnt be asking this question.

Yes there are ways in which fuel can be stored to last much longer, but do you want to risk blocking up your fuel system for the sake of a few quid.:).......Bin It ! dont even think about putting it in your car as that could cost you even more !
 
If you put it in your car, its diluted with fresh fuel and doesn't cause a problem. Unfortunately we don't have any petrol fueled cars anymore, so I'd have to give it away.
 
Little Sam said:
"Old" fuel is anything over 30 days...there are ways in which fuel can be stored to last much longer, but do you want to risk blocking up your fuel system for the sake of a few quid.:).......Bin It ! dont even think about putting it in your car as that could cost you even more !
Click to expand...

I cross my fingers before attempting to start my dad's lawn mower...it's generally many months since it was last started, and sometimes it's years since the fuel was new. Plus, the petrol was sold to my dad in an old motor-oil can! Thumbs-up for recycling, but that can't help combustion, can it?

Credit where it's due, though...the mower (US-made, under the name 'Eden Park') always does start. Must be a very low-tech fuel system, no?
 
I'll state again dont go near your car with old fuel.

Varnishing petrol will clog your EFi system and your fuel filters and youll be wondering why your car wont start. Or if it starts why its only running on a few cylinders. It does not dilute once its turning to jelly, it only floats about in lumps waiting to be sucked in and cause problems.
 
Little Sam said:
I'll state again dont go near your car with old fuel.

Varnishing petrol will clog your EFi system and your fuel filters and youll be wondering why your car wont start. Or if it starts why its only running on a few cylinders. It does not dilute once its turning to jelly, it only floats about in lumps waiting to be sucked in and cause problems.
Click to expand...

the old polo must be awash with jelly by now then....
 
>"Varnish".....

You can stop the varnish effect with an additive. One problem with small two or four stroke engines, including outboards, chain saws and mowers, is the spark plug tends to varnish inside the plug if they are not used regularly, which most aren't, and need a new spark plug. I had to replace the chainsaw and mower plugs every year but then discovered the additive, which I bought from the local chainsaw and mower shop. As a rule of thumb cars don't get varnish because they have a much bigger petrol through put and are used regularly.
 
I hadn't riden my Kawasaki for about three seasons when I fancied putting back on the road. To my horror, the tank fuel had morphed into a horrible thick gue. New fuel wouldn't touch it, neither would acetate. Caustic soda cleared it, eventually. The tank switch and filter systems were ruined. Fortunately, I always drain down before putting it away for hybernation. I had the bike for 20 years and this was the catalyst for my selling it last year. I've played with all maner of engines all of my life and never experienced anything like it before.

I still have half a tank of 4-star, leaded in a car in my garage, unused since about 1994. I wonder what awaits me when I finally decide to put it back on the road.
 
Ah well, sounds like it's not just down to oldish fuel; and as perceptively pointed-out above, I leave carburettors well alone. For some reason even though I'm reasonably good at mechanics, those twiddly little screws, needles and jets conspire to defeat me every time. Hence why I get someone else to service my outboards, flymos etc. Good old diesels!!

Thanks one and all.
I'll get on the Dog and Bone to "Man with a Spanner" and let him have a look at them.
 
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