Does petrol really go 'off'?

[p.dath]

I know that keeping petrol for years on end may cause the liquid to partially evaporate, leaving a solid cake behind (I've seen it before in a bike that hadn't been ridden for 10 years) but, will petrol lose some of its octane or otherwise degrade after a period of several months?

I'm only asking because I'm planning on buying enough fuel for the lawnmower to last 6 months because I don't like going back to the petrol station with a petrol can every couple of weeks.

The Octane of fuel constantly drops. When you buy petrol it usually says something like "not less than "x" RON" or similar. When they manufacture the fuel they make it with a higher octane rating. You can loose 1 or 2 off your octane in a week. So service stations often have weekly fill ups.

The cost of petrol is cheap compared to a fucked engine. I wouldn't buy a six month supply. And I wouldn't use six month old fuel.
And if it is pre-mixed two stroke I would be even more carefull.

I'd be thinking twice about even using 3 month old fuel.

[MSTRS:Me]

I'd be thinking twice about even using 3 month old fuel.

Why? My mower has been more than happy on old fuel for about 13 years. Still is.

[p.dath]

Why? My mower has been more than happy on old fuel for about 13 years. Still is.

I know someone who wrote off a chainsaw same way.

Personally for me, I'd rather shell out an extra $5 for new fuel and not worry, and than extra $500 to $1000 for a replacement lawn mower.

[Banditbandit]

Can be especially bad in boats with underfloor tanks. I put in $20 or so of 120 octane Race Gas (Challenge Pukekohe has it) to refresh the fuel before each summer season. That and a bottle of Yamaha Ring Free to burnt the carbon off. Seems to work well for me.

In sealed containers or fuel systems it's not so bad. Boat underfloor tanks have a breather and all the aromatics / volatiles evaporate off over time.

My outboard mechanic also told me that it's a worse problem for 2 strokes, 4 strokes seem to be able to handle it better. He approved of my Race Gas use as well.

If you're boat's stting around not used long enough for the petrol to go off ... you are not using it enough ... sell it to me ... I'm in the market for a one ...

Same for bikes .. if you're worried about petrol going off in a bike's fuel tank you're not using the bike enough ...

[Wannabiker]

Your mower will be fine. For Briggs and Stratton, the lower octane fuels are fine. Mine runs on anything from avgas, old 2 stroke mix, 95, and 91. I have even burnt up some old Model aeroplane glow fuel through it (methanol).

My chainsaw sits under the bench for months on end, and goes just fine with whatever 2 stroke I have on hand...usualy 32:1 from the dirt-bike.

[DangerMice]

After buying the scoot, my car sat in the garage for a year with half a tank of gas till I sold it. When the buyer started it up, it puffed out a nice cloud of smoke out the back, then it ran fine. I have no idea if it is *still* fine tho.

[p.dath]

Someone who is an expert will correct me on this one.

Basically the RON is the resistance to knocking in the engine. The RON rating of petrol reduces every week it is in storage (I understand loosing 1 to 2 RON a week initially is common - higher octane fuels "go off" faster).

So basically, the engine is far more likely to start knocking if you are using old fuel in it.

[steve_t]

Interesting read...

http://retrorides.proboards.com/inde...y&thread=88667

[MSTRS:Me]

Someone who is an expert will correct me on this one.

Basically the RON is the resistance to knocking in the engine. The RON rating of petrol reduces every week it is in storage (I understand loosing 1 to 2 RON a week initially is common - higher octane fuels "go off" faster).

So basically, the engine is far more likely to start knocking if you are using old fuel in it.

It's all a matter of the correct advance for the octane you are using.
Don't you remember having the dizzy on the old Zephyr marked at 2 places of advance? Put it here for Super, and here for Regular...

[HQfiend]

Someone who is an expert will correct me on this one.

Basically the RON is the resistance to knocking in the engine. The RON rating of petrol reduces every week it is in storage (I understand loosing 1 to 2 RON a week initially is common - higher octane fuels "go off" faster).

So basically, the engine is far more likely to start knocking if you are using old fuel in it.

RON stands for Research Octane Number which is discovered through laboratory testing at a chemical level. Basically means they (the lab boffins) think that it'll resist knocking at that level. Theoretical Octane Number would be a better name for it!

MON stands for Motor Octane Number and is found by actually running a special "donk" motor that has variable compression and a bunch of sensors all over it to actually measure when the motor knocks. I.E. a true octane number.

As we will have all discovered if using older engines that the numbers at the pumps are usually the RON numbers and in reality the MON is between 3 and 10 octane points less! So your 98 RON is in reality about 92 - 93 MON.

With leaded gas it used to be the average of the MON and RON.

With AVgas they don't like to take chances with fuel quality as you can't get out and push if the prop stops going round! They will sell off the fuel due to age and also water or particulate contamination, or a dozen other reasons, not just "the octanes gone off"

To answer the original question. If the gas is sealed in an approved storage container the fuel will stay stable and be usable for several years.
The paint thinner they laughingly call petrol will only degrade if exposed to air and allowed to evaporate.

[scumdog]

I'd be thinking twice about even using 3 month old fuel.

Whatever bike I put on 'hold' ends up with fuel older than that, never had a problem.

Likewise major work on one of the V8s can see them sitting for up to a year.

They've always started up OK afterwards.

Maybe the colder climate down here helps?

[Flip]

What the hell does octane have to do with stale fuel????????? Poor old Mr octane gets blamed for a lot of things now days.

Pump gas goes off with time because the nice easily spark ignited small hydrocarbon molecules evaporate and leave behind the big nasty hard to ignite large molecules. The small molecules are partially soluble in to your plastic polyethylene petrol can so are slowly lost over time. However I live rurally and leave gas in a plastic can for ages (years) and it still works fine.

Just a hint, smell the fuel, its the small aromatic hydrocarbons that are lost, if the gas smells oily and not solventy its probably old fuel.

[Skyryder]

You can buy an additive that prevents deterioration. Not too sure of its name but yu get it at chainsaw, lawn mower outlets.




Skyryder

[scumdog]

You can buy an additive that prevents deterioration. Not too sure of its name but yu get it at chainsaw, lawn mower outlets.




Skyryder

Yep, a lot of classic car owners who don't use their cars very often use the stuff.