How long does petrol last?

Not sure I believe that.

There certainly was a time when it was worth giving your lawnmower "new" petrol at the start of the season, because it does contain a range of boiling points and the more volatile ones which help starting do evaporate. Also remember it from my motorcycling days where sometimes you would recover something that had been dumped in a barn for years.

Less of a problem now, I think, because magnetos have better magnets and many systems have some sort of electronic ignition, giving much better sparks. I have several petrol devices that are often left six months or more and I've never had real problems with starting.

Also of course we've standardised on low octane petrol. the SD1 might be in the group that preferred four star.

PFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT!

:)

I learned about petrol at a young age. No long term damage, and my eyebrows have grown back.

Bollocks....

Tim

Depends on how it's stored. If it's in a sealed container, it's ok. Had someone left the filler cap off?

Did the engine run ok when the petrol was replaced?

after a few years it smells wrong, but still runs, but can be difficult to start from cold, worse thing is that it tends to gum up small jets and float needles. So if the sd1 was carbed then old fuel could be a real problem, less so if it injected.

Will it fire on easy start?

Fuel condition depends on type of vent system on fuel tank.

Modern cars (late 80's) that have evaporative control stuff like charcoal cans have a system with gulp valves and spring loaded vents. The gulp valve in tank cap will gulp a slug of air if the tank pressure is too low. The vent will only vent when pressure is too high. Both protect the tank from collapse or bulging. Once an equilibrium is established very little air gets drawn into the tank and it's more likely that vapour is expelled to the charcoal can.

Older systems would simply vent though a pin hole in filler cap. Every day it warms up the vapour expands and pushes out of the tank, every night it cools and air is drawn in. The air contains water vapour, it condenses on the tank wall and runs down to the bottom of the tank below the petrol. Due to pick up at bottom of tank it is then pumping water from the tank.

There are microbes that can live at the fuel water interface and they are a real problem for many classic car/bike owners. They eat fuel, piss water (wouldn't be so bad if they pissed alcohol like yeast), a little bit of water becomes a lot and a layer of slime. There are treatments that sterilise the fuel tank and system.

You can't always get petrol to ignite with a match.

I find that hard to believe. You can?t light it with a glowing cigarette end but surly an open flame is more than enough?

Tim

ah, it must be a lit match, that was not specified :)

Are you related to Graham?

Every year, for the past 30 years at least one of my fleet runs on 6 or 7 month old fuel every spring, now its usually BP Ultimate but sometimes it's been Sainsburys / Tesco 95 unleaded.

I've never noticed any issue with running on this 'old' fuel

The car is usually laid up in October with a service, MOT and fuel fill to the tank neck. Following washing and 'winterising' then other than the engine and gearbox being turned over with a socket on the crank pulley every month or so it remains untouched until the next proper engine start and road run the following April or May. Usually a first time starter.

My old Royal Enfield 'Bullet' 350 (well, it wasn't that old but came from Madras and from an old design) was left unused for over 3 years in a lockup garage. The remaining petrol stunk like paraffin, the carb was a bit gummed and the battery was nearly dry, but once 'petrol' was flowing it started on the second kick.

So if it's (still) volatile enough to start with a spark (all be it under pressure) I can't see why it wouldn't light with a flame?

Cheers, T i m

Needs the right air / fuel mix. The match lights the vapour, not the liquid, so if it's not vapourising (stale fuel may struggle with this as the lighter components will already have vapourised), it won't catch.

That's not a huge issue with petrol - diesel is a massive issue due to the increasing bio content. We de-sludge diesel tanks every year for this reason.

Indeed. Then petrol doesn't burn. Its vapour does. There is a 'trick' involving a lighted cigarette and an open container of petrol which I've seen....

Petrol left in a fuel system can 'go off' and cause problems - our Outlander has some kind of warning system if it detects you've not run the petrol engine and 'old fuel' is sitting in the system- but the issues are more 'subtle' than not burning.

I have an 2 stroke generator I rarely use. If I give it a squirt of 'quick start' it will start with petrol that has been sitting in the tank for well over a year, maybe 2.

Briggs and Stratton recommend adding a petrol additive for their lawnmower engines which is meant to: "maintains fuel stability for up to

3 years."

Since starting to use it I've not had problems with starting with petrol that has been over-wintered. Previously, on an older petrol mower, I've had to strip the carb and flush through with fresh petrol before it would even attempt to start.

When I was in the services, we had petrol electric sets that were used to charge a bank of nife cells for lighting, there were a few requirements in the SOP for the use of the pe set, including that it must always be run dry before storage, and turned over till on the compression stroke. I have always done that with my petrol mowers for winter and with a refilled tank they always start easily. Running dry means the fuel tank does not breathe damp air in and out, same advantage for the valves, spark plug and piston group, as the float chamber is empty the needle valve is left open, and the lack of fuel means the jets remain clear of the gum that would form with a small amount of petrol evaporating all the good stuff and leaving the junk. (and some B and S engines have a vacuum fuel pump that easily gets stuck with old fuel.)

I?ve been thinking about this, but my B&S lawnmower starts fine every spring (for the last 20 years) with last year?s fuel.

Tim