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Old 01 September 2005, 08:59   #1
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Fuel Degredation

This could spark a few disagreements but your opinions would be good....

A car mechanic & former outboard owner recently agreed that unleaded fuel degrades quite quickly & that I'm right in not using fuel over 6 weeks old. However, he then went on to tell me that, as my engine doesn't inject oil, & I premix, the problems caused by bacteria don't affect premix in anything like the same way. Apparently my premixed fuel could be good for months, maybe even a year. I'm surprised but it seems to be a good discussion point that would save me quite a bit of cash & hassle. I cry every time I have to thow away petrol at the price we pay today. Anyone got any opinions on this?
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Old 01 September 2005, 09:40   #2
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Good Topic!

I have heard similar stuff like:- "If you are gonna keep petrol in a tank for a while, the tank should be topped up (as little airspace as possible) and the vent closed." While I can see the logic in this, in practice whatever is left in my (removable steel) tank after an outing, simply gets topped up when next I take the boat out. This can be a matter of days, weeks or even months over the winter! Sometimes I forget to close the vent too. My fuel is 50:1 premix, and my motor is NOT ultra-modern high tech. Petrol is far too expensive to throw away, and I don't have any fuel-type problems with my motor, so "If it ain't broke - don't fix it!" works for me. My mate treats his fuel the same way (his boat is kept next to mine) except that his is raw petrol (no premix), and he has had no problems with this either.

Just my experience - anyone else?

Bill
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Old 01 September 2005, 14:24   #3
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Quote:
Originally Posted by Bill S
Good Topic!

I have heard similar stuff like:- "If you are gonna keep petrol in a tank for a while, the tank should be topped up (as little airspace as possible) and the vent closed."
This goes back to aviation practices, where they have found that keeping the tanks topped off minimizes airborne moisture vapor condensing on the tank walls and collecting in the bottom.

Does it help in a boat? Don't know. Can't hurt, though, unless the boat is laid up for a few months and all the fuel goes bad from aging.

jky
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Old 01 September 2005, 15:48   #4
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Here goes:- Unleaded does deteriorate more quickly than leaded. Unleaded begins to go off after about six weeks, leaded six months. The problem is that the evaporates (the volatile, burny part ) tends to evaporate off, leaving the fuel less effective. This is easily cured by chucking some fresh in with it. Over the winter it is good to leave inboard tanks fully full as it prevents condensation and corrosion in the tank. Fuel storage stabiliser should then be added at the required amount to prevent the fuel from going off. This is why so many bloody lawnmowers refuse to start at the begining of the summer. We march them off to the garden machinery guy who changes the fuel and the plug, smiles and holds his hand out for 60.
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Old 01 September 2005, 16:11   #5
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I posted a question on this a few months ago about petrol I had for about 18 months in outboard tanks filled to the brim.The general consensus was that it went off after a few weeks, ie. the most volatile compounds which helped to start the engine were the first to evaporate off.
I used the petrol in my Mariner 60 and the only problem I encountered with the first tank was it took a while to start- the engine had been idle for about four months over Winter time.Subsequent tanks of the old stuff burned off without any problem.
An ex-army chap I know told me they used petrol which had been stored for years in jerry cans without any problem and I can confirm my old Honda XL500 enduro which I sold 3 months ago fired up on the second or third kick on leaded petrol in it's tank which was filled last in July 1997!
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Old 01 September 2005, 17:06   #6
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An XL500 would run on a wet fart!
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Old 01 September 2005, 17:16   #7
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The comments about pre-mix are right in my experience. I would leave fuel in my tanks (half full, full, whatever) with the 100:1 oil ratio in it from september to may and it would start no problems when I took the boat out of storage. I would guess that there may well be problems with pure unleaded.

I also got a little water in my fuel once. Made the engine splutter, but I just put extra fresh petrol in and ragged the boat around the solent and it burnt it through nicely...not shore if that was the best thing to do, it was only a 1987 25hp Yam carb model!
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Old 02 September 2005, 04:36   #8
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what about adding 'fuelset' or similar additives to petrol which might not be used soon (ie... to our tanks from now on...) - these claim to prevent biological contamination - shouldn't these make the fuel last longer ???

as said above - keeping the tanks filled prevents condensation and corrosion and prevents a buildup of water in the tanks which really makes an engine hard to start ;-) - and can damage an injection pump - I dont think fuelset is an alternative to this - but it may help protect the full tank of fuel from degrading...

it's really hard to imagine any kind of biological life forming in petrol or diesel (the must be nicer places....) - but I know it happens...
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Old 02 September 2005, 10:33   #9
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Thanks all,
Great to have all the opinions. The general view seems, top it up, mix it up, seal it & shove some additive in if it's going to lay a while. However, do we have any scientists out there who can answer the point in laymans / boatmans terms? If only just to confirm or deny the points raised so far?
Cheers
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Old 02 September 2005, 10:41   #10
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I think that disposing of six week old fuel is a bit extreme!

What do you do with it?
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