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Seahorse
16-09-2009, 08:43 PM
I havnt been out for about 6 weeks.
last trip i topped up tank. 100 lit.
I used about 30 litres.
Will it be ok to just top up with unleaded or should i use premium if i go out over weekend.
Just worried about fuel sitting for 6 weeks

cheers
greg

lethal098
16-09-2009, 08:53 PM
greg, these days unleaded fuel does not go off that quick,just top back up with standard it will be fine, putting in premium will not lift the octane rating of the fuel you currently have as they will not mix together. cheers Lee

boney-leg
16-09-2009, 09:48 PM
If you're worried, try a fuel conditioner ...

FNQCairns
16-09-2009, 10:24 PM
I have 4 20L Jerrys all lined up like soldiers, within a few days of the last trip I evacuate the tank into these. It seems this last couple of years the Jerrys have paid for themselves quite a few times over as i don't seem to get out much at all and who knows could have saved me a rebuild...I have seen a few ruined engines from old/bad fuel in my time.

cheers fnq

Angla
16-09-2009, 10:45 PM
I think 6 weeks will not hurt it too much

I normally use 95 octane to be more sure there is no Ethanol in the blend.

Cheers
Chris

RTFM1000
17-09-2009, 07:13 AM
Hi, Yamaha produces a fuel stabilizer & conditioner that "Keeps fuel fresh and potent during prolonged storage up to 1 year. Prevents gum and varnish deposits. Draining fuel for storage is not necessary with the use of this product."

I use it in my generator which may sit for quite some time between uses as well as in my outboard to keep the fuel system clean. (Unfortunately, the outboard may sit for some time between uses also).

Regards. RTFM1000

cormorant
17-09-2009, 10:07 AM
Conditioners cannot refresh fuels and must be used on fresh fuel as it is added to the tank to get good results


There are different types from ones that create a "skin " ( not a actuall skin but a chemical one) barrier on the top surface that prevents evaporation of volatiles and those that mix through and attache themselves to various parts of teh fuels structure . Some are suitable for temperature change situations and others for where a moored boat is moving the fuel in the tank all the time. Some stop degredation and settling due to either hot or cold conditions.

Some conditioners are just a octane booster and that is not always a good thing especially if the motor it is being used with is under high loads like a outboard. Those octane boosters can free up varnish in carbs , melt hoses and cause all sorts of issues on older motors.

I would speak with the fuel company you buy fuel from, the motor manufacturer and ask them what one is suitable as I have seen issues with some imported ones designed for different countries. The fuel companies have conditioners that they add at production for transport , marina use and remote area fuels. Some are avaliable from them but possibly only in bulk.

lunchcutter
17-09-2009, 07:19 PM
what they say is add 10% new fuel should be fine

rowanda
17-09-2009, 09:03 PM
greg, these days unleaded fuel does not go off that quick,just top back up with standard it will be fine, putting in premium will not lift the octane rating of the fuel you currently have as they will not mix together. cheers Lee

and where did you get the information that premium will not mix with normal fuel??????

oh and 6 weeks is not old, it will be fine, top up with fresh normal or premium if you are worried, it will be fine

BM
17-09-2009, 09:18 PM
Adding fresh fuel to stale fuel is a recipe for problems.

As fuel breaks down it produces soem harmful oxides. These oxides cannot be counter-acted by adding fresh fuel. Its a myth!

Mercury claim that fuel begins to degrade within 2 weeks. By 8 weeks, its rubbish and should be discarded or cautiously used in your car (diluted) or in your lawn equipment (diluted).

Mixing premium fuel with stale fuel WILL increase the overall octane rating of the fuel, but that is really not relevant considering the harmful bi-products of the already degrading fuel.

Cheers

PADDLES
18-09-2009, 10:27 AM
is stale fuel really much of an issue with modern "newer technology" 4 stroke motors? i mean, i've left fuel to sit for 12 or so weeks with my suzuki 4 stroker and the thing just started and went like a champion when i wanted to use it. i wouldn't know if there was some form of internal damage or additional wear caused by this but it certainly didn't seem to make much of a difference. i've now just bought my first ever mercruiser, it's an mpi model that's basically an automotive engine, i have never had an issue with stale fuel in my car so am expecting this one to be the same. pre-mixed 2 stroke fuel however, totally different story.

FNQCairns
18-09-2009, 11:02 AM
Yeah paddles stale fuel impacts the valves usually gunking the stem up or in worst case scenarios burning them out. Your car has a more or less closed fuel circuit boats do not, this is a mojor part of the riddle that many forget when the state how long fuel lasts...dontthink of a jerrycan withthe cap up tight think of one that is missing it's cap, then give an estimation of Time versus Quality.

Hey congrats on your new engine hope there is a boat with it:).

I was not long ago very interested to see what was going to be the upshot of the "unleash the beast" segment on a TV fishing show, they swapped out a mercruiser in a 23 bertram for a 350 v8 yamaha.

Outside of all the marketing ...the upshot without the marketing just the graphs showed that in nearly all areas except for noise the mercruiser was fully or very nearly a match for the V8.

All this was after the first run where they bagged the daylights out of the mercruiser, then they proceeded to fit an engine to this style of boat that although greatly quieter also lived up to the same bagiing but never got it.

We haven't actually come a great deal further it seems.

cheers fnq

toecutter
18-09-2009, 01:20 PM
Hey, I've got some questions:

- Will fuel stay fresher longer if a tap is installed on the (underfloor) tank breather so it can be closed off when not in use?
- will a full tank (no air layer above) stay fresher then a partially full tank?

cheers

yellowbeard
18-09-2009, 02:08 PM
Just on the non-technical record, I left my 115hp Merc sitting for well over 6 months three quartres full before I sold the boat, and I was really worried about her starting. But she started straight up and ran like a train. On a technical note though I know it's not good practice at all.

FNQCairns
18-09-2009, 02:34 PM
Hey, I've got some questions:

- Will fuel stay fresher longer if a tap is installed on the (underfloor) tank breather so it can be closed off when not in use?
- will a full tank (no air layer above) stay fresher then a partially full tank?

cheers

Yes but i wouldn't do it, the potential pressure build-up with minimal fuel in it might surpass the tank and connections original design.

You could fit a valve in line with the breather but then again it could get blocked.

and

Yes full tank is the way to go for 2 reasons, it decreases the available surface area for the fuels volatiles to flash away and it increases the mass of fuel to help starve of temperature extremes which in turn increases volatiles escaping as well as oxidation of the fuel.

Full tank and some conditioner is the way to go over any 8 week period if you must.

cheers fnq

PADDLES
18-09-2009, 08:53 PM
thanks for the info fnq. good one, yeah the mercruiser's attached to a big piece of fibreglass. interesting point about the full fuel tank aiding life, i always used to keep my tank full to have less air for condensation to form, this must have helped me with my fuel not seeming to go off.

Flex
20-09-2009, 11:00 AM
I realise fuel loses some of its octane rating over time so your energy yeild will be slightly lower.

But can someone explain to me how old fuel can actually damage an outboard? and is it more dangerous for injected 4 strokes etc compaired to 2 strokes?

I have a carby 2 stroke and often leave it in my tank for 6months +(real pain to drain it) and I haven't had any issues at all.
Does the oil mixed in cause any issues aswell?

banshee
20-09-2009, 11:57 AM
My understanding (very limmited) is that old fuel loses it's stability and becomes more volatile,the downside to this is that the fuel predetonates before the piston reaches TDC this in turn leaves the piston,driven by huge amounts of inertia,trying to compress an already expanding mass of gas,the end result is often parts failure..........but I could be wrong.

cormorant
20-09-2009, 06:44 PM
light volatiles ( things that go bang) in the fuel evaporate off and you are left with heavier oils , varnish and waxes

The heavier oils burn slow and hot with lots of deposits rather than than go bang in the combustion chamber

It can affect 2 and 4 strokes

Depends on how loaded the motor is and how well it is cooled and lubricated as to what damage happens

Hot slow burning oils burn holes in pistons and stems or seals off valves while screwing spark plugs. Deposits on cylinder walls and lean running can scuff piston skirts.

Waxes and varnishes can play havoc with hoses,carby bowl, needle and jets and with fresh fuel or "cleaner:" can be dislodged and become mobile and clog injectors or pre filters.

When motors are heavily loaded they basically can't get the heat out of them. With normal fuel the "bang"produces the HP and without it the motor labours and the natural reaction is to use higher throttle and hence more fuel and heat for same performance.

I have seen motors get that hot they have run in diesel mode where the heat ignights the incoming fuel and the motor can't be switched off even when you pull the plug leads

Have seen holes in pistons, screwed heads , scuffed bores, clogged exhaust ports and heaps of fuel delivery issues . Some fatal some not nut none are good.

A lot depends of storage of fuel.