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Outboard Motor Questions - Stale Fuel / Fuel Maintenance


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Hi Raiders,

I have some questions on boat fuel that I'm concerned about myself and I know that the right answers to my questions will be of assistance to other members who own a boat and for whatever reason are not in a position to actually go out on the water and run the boat's outboard motor under load as often as they should.

My questions below relate to fuel maintenance and the risks involved in using stale fuel and I would like to hear your opinion on this and look forward to your advice.

I believe there is an additive that can be used when the tank is initially filled up with fresh petrol petrol?? Are any of you experienced with this and is an additive effective in preventing the components of the petrol (and the 2 stroke oil when premixed during the run in period) from breaking down and damaging the outboard ?

My inbulit tank hols around 40 liters and at the moment it would have around 30 litres in it plus added 2 stroke oil to increase the ratio together with the oil injected oil to 25/1 for the 10 hour run in period

As some of you may know I purchased a brand new 60hp Mercury bigfoot motor in July of last year from Huett Marine to replace an underpowered 35hp Evinrude in my side console boat but apart from running the motor on the water for about an hour or so (with added premix oil in the inbult tank to make "the run in mix" 25/1) for the first 10 hours run in time, I have not taken my boat out for several months whilst waiting to have my hip joints replaced, an operation which was scheduled to take place in August of last year but had to be postponed due to my picking up a golden staph type bug in my local hospital where I was admitted and kept in for 6 weeks prior to the proposed hip joint replacement surgery to be performed at another hospital. I was admitted to my local hospital because of unbearable pains in the buttocks and legs which resulted in extreme pain on movement and an inability to put weight on my legs due to time delay and the eventual deterioration of both hip joints. Thought I'd elaborate a little as I haven't been as active on the site as often as I would like to be.

Below are some comments from a boating company in Victoria on the dangers of using stale fuel highlighting the seriousness of running stale fuel in an outboard motor.

I have harped on about fresh fuel and good quality oil (for 2 strokes) for a very long time.

Unfortunately, we all have a bit of a mindset when it comes to petrol. We don't want to toss it out because it costs "so much".

Our perception (probably as a universe!) is that petrol is very expensive. BUT, people will pay $1 per litre or much more for BOTTLED WATER, $2+ for a litre of Skinny milk, $24 MINIMUM for 3/4 (50mls less) of a litre of Bourbon and so on it goes...

Sure, we don't use as much of these other products as we do fuel but it is a moderating thought.

The mental mindset of not discarding your boat fuel based on what it cost is what costs people much much more when their engine shits itself.

Stale fuel is a VERY bad thing but people generally do not understand that. They often think its the mechanics "excuse" for why their marine engine died. They need to understand the fuel lifespan and usage constraints.

If you need to lay your boat up for a few months and particularly if it has an underfloor tank which has a lot of fuel in it, then apply a fuel conditioner whilst the fuel is fresh and it should still be usable when out of hibernation.

But over and above that, fuel management is the key.


An engine can run very easily when it is "free running", ie: no load. Unfortunately, a no load "all ok" is often reported and followed by a "won't run under load" scenario.

First up, I would change the fuel (is it 12 months old too??) and try again. Fuel dies after about 8 weeks (faster in warmer climates than Melb). Stale fuel is the number 1 killer of marine engines.

Then I would be looking at the fuel system for blocked jets (as mentioned) and then looking at the ignition system.

I believe there is a fuel additive available that keeps petrol fresh when added to new petrol ??

Have any of you had outboard motor problems caused by the use of stale petrol ? - I think the last thing I would do is to run my 10 months old fuel in my brand new Mercury Motor so I had best have it drained out then make sure that I run the motor regularly.

Should a certain amount of fuel be left in the tank to lubricate the carburetter??

I would appreciate as much advice that I can get on the subject of stale fuel and how you go about your fuel management.


jewgaffer :1fishing1:

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I'll also be interested to see the replies to this Jewgaffer-I'm in a similar situation,I have a 115hp Merc 2 stroke with i/2 a tank of year old fuel due to illness that turned out to be bowel cancer,since removed, and despite mop-up chemo I'm feeling the enthusiasm returning, but have the same concerns about running the old fuel.It's terrific to see you posting again,I hope you're up and about as soon as possible.



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The following may not specifically answer the question but rather provide some background information..

A year or so back I contacted the 2 fuel companies & asked the question on shelf life. The bottom line , there is no definitive answer as it is very much dependant on the container storing the fuel.

Petrol stored in a sealed drum could last a year or more where as if left in the open & exposed to air it may only be a week or so.

For boats , as with cars with an inbuilt tank there is exposure to the atmosphere via the breather tube. As the temp of the day increases & decreases so the fuel expands & contracts moving vapour through the breather tube.

How long will fuel remain in good condition , there are so many variables , temperature , hose dia. the amount of fuel in the tank etc.. , who knows.

A search on the web , Petrol shelf life , or Stale fuel damage or similar searches provides lots of sites relating to the subject.

There are claims the stale fuel will damage pistons but I've not seen a post on any website saying the motor / pistons failed due to stale fuel. That's not to say it may not happen but it would be nice to see some positive evidence.

I think if the fuel is really stale , the motor won't start or if it dose would run rough or struggle to reach max revs.

I think the bigger issue is fuel left it carbys progressively evaporates leaving an oily film & blocking the jets.

So , where does that leave us. Personally I think any fuel over 6 months old in an inbuilt tank is becoming risky , how risky , that's the $64 question


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Great topic Byron. Good to see you posting again.

I found this info about fuel very useful as my boat has been out of action for about 4 or 5 months and will still be off the water for another month or two until the motor has been serviced.

As I don't have much fuel in the tanks, I guess the best option is to get rid of it.

I'm interested to hear what anyone else has to say.



Hope you're still improving healthwise Byron and that you'll be back on the water soon.

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good to see you back jewgaffer.i steer clear of any ethonal blened fuel so i only use 95 (ron) premium . i know the Americans swear by this additive called sta-bil which helps with low ron fuel and ethonal blended fuels.it claims it keeps fuel fresh for longer and takes out any moisture created,and you only use 30ml per 9.5litres of fuel I think i saw it at the big boat store the other day but i will check 2morrow.(it is not starbrite though) and i also have a 60hp merc bigfoot but mines a 4stroke cheers dunc 333

Edited by dunc333
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I remember reading somewhere that Mercury consider fuel over three weeks old stale.

The Stabil product is popular in the States but I also read they have a blue "Marine" Stabil and not to use the red Stabil which is the only one I've seen here. Mercury/Quicksilver do a product called Quickstor for longer storage periods and Quickcare for per tank use that help condition the fuel. No doubt BRP do similar products.

Do they work...difficult to say but there is great support for the Mercury products in the States.


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Hi Guys,

i only ever use fuel in my tank once. unless i will use it again within a week or two, i just drain the tank and use it in the cars as they have knock sensors to compensate for octane (i beleive this lowers quite quickly in premium fuels). sure it can be more costly than using the existing fuel in the boat again but given the extremes our motors already face (salt water, wind spray etc) i guess its purely for my own peice of mind. Never once had a problem with being stranded because of old fuel or blockages etc so i will continue doing so..



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I believe there is an additive that can be used when the tank is initially filled up with fresh petrol??

There is an additive called Stabilo that was recommended to me by a few friends. I bought a bottle but am yet to use it. Not much help I know, but at least it's a brand you can research.



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Not actually answering your question but if you are able to remove the fuel from your tank without much hassle why not put it into your car if possible. When I had my boat I often did this and had no problems with the running of my car. The bit of oil that was in the petrol did not effect my car one bit. I would time it when I was filling up. It was easy for me as I had portable tanks.

Even today I worry about what the quality of the fuel I am buying for my car regardless of where I buy it.

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This might be a coincidence but usually it takes me a while to get my oil injected 70hp yammie running after it has been sitting for a month or more. Today, knowing the tank was quite low I ran it down to the servo and popped 35 litres of 95oct (there was no std unleaded) in it. Took it home and started almost immediately. So I'm not 100% sure, but I think the fresh fuel made a differance. The only other thing I did differant was I gave it a few extra squirts with the hand pump before turning the key. I made sure the bulb was hard. Normally I'm reluctant to to pump it too much as once I went overboard with it and fuel started pissing out from some over flow on the outboard and when I eventually got it started there was smoke everywhere. I've gone off topic now.

My tip would be to try and keep it fresh. I won't be putting too much fuel in the tank from now on, but the more air in the tank the more chance of water forming in it also so I can't win.

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the problem with old/stale fuel is that mositure is the fuel cause the octane rating to drop,

this can cause the engine to ping, and worst case blown engine.

i find that if you keep the fuel low, and top it when you needed it, the motor runs alot better with the new fuel in it,

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Ethanol vaporises faster than petrol, and to make matters worse it more rapidly absorbs moisture. Ethanol petrol should not be used in boats or lawnmowers due to that the fuel may not be emptied frequently and/or topped up.

Sent from my iPod touch using Tapatalk

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Just a thing on the "shelf life of fuel" I used work to on a product tanker than ran the Australian coast and on a number of occasions we would load ULP first, then a day of two later when we have completed loading other oil based products. All the fuels would be tested and on a few occasions ULP would be off spec so it would have to be pumped ashore and re refined. This could happen in periods of less than 30hrs, but on other occasions it would be good for days. There are so many factors for why this occurs but some of the main causes are temperature change and humidity.

When it comes to running your outboard on it, majority of the time it will be fine but it does increase the chances of something going either slighty wrong or majorly wrong. It can be a fine line. As was perviously mentioned fuel is cheap compared alot of other stuff we purchase even if it is over the $1.50 mark/L. Would you risk you $5000 - $30000 outboard for the sake of $15 - $50 of fuel?

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