Quick question on fuel

[marshan162]

What is the effect on using premixed old fuel, a year old. Started the engine today on fuel in tank from last year, and it wont idle , plugs are wet with oil. Does the oil seerate and colect at the bottom of the tank and does the the fuel lose its ability to to ignite easliy

Thanks
Karl

[Travis Fulton]

old fuel goes stale, and sunlight has its effects also! i would discard it and mix fresh , a-lot cheaper than a re-build

[Dean Dorsey]

If motor is carbed and the old fuel sat in the carb bowls, a green slime will form in the bowls that will clog the jet orifices and prevent the good stuff from getting by can acct for the lack of idle. You may need to pull bowls and jets clean and try again.

[marshan162]

I had the carbs and pump apart a few weeks ago and cleaned them and checked every jet. In hindsight I wished I had tried starting it before I did this. I don't intend to run it in ernest with the old fuel but was just curious if the idlng problem could be caused by the old fuel. You guys probably don't realise that Gas and oil comes to $8 per UK gallon here, so for testing in the dock and setting up there is a big temptation to use old stuff.

Karl

[quickonstep]

its expensive, but.. think about this.. 8 uk dollars per gallon vs.. 2000+ for a rebuild... the choice is yours

Jesse

[marshan162]

I agree, I wasn't going to put it it in the water with the old fuel, just running it on the drive. Thats $8 US but UK gallon

Karl

[Rickracer]

...much less able to burn consistently enough to produce a good idle. :( Sorry, dump it and start fresh. :cool:

[Fish]

Can old fuel/bad fuel blow your motor? If so, what exactly causes it to grenade?

[chris_lacey]

don't know about UK, but here in Canada, alcohol is used to increase the octane. When the alcohol evaporates along with the rest of the "high end" volitile components of the fuel, it actually loses octane (rating), becomes more detonation prone and will melt pistons.

[sho305]

It sure does lose octane. I have stored cars that run terrible after a year, spark knock bad. I put premium in them to last longer, and my boat too. I get it in the last tank if I don't normally use it and it seems to still run in the spring ok except for one thing...the carbs can react depending on the metal they use. So I drain the bowls, and lawnmowers love to do this as well. The rest of the fuel is ok for 6 months if no moisture gets in it, but I sure would not recommend running hard on it. I normally use it in my lawnmower. Usually leave a couple gallons in the boat and pour out if more; then I add fresh fuel right away. If it sets more than 6 months I clean carbs. With hipo stuff, I'd be more careful. The oil also reduces the octane of the fuel in a 2 stroke. I make it a point in the fall to nearly run all the fuel out because it is a pita to drain the tank down.

Bad fuel does not burn right, and can leave more deposits or detonate or gum up carbs and tanks. Very bad on a hi output motor like a 2 stroke OB, not much of a problem in the understressed lawnmower.

Of course with a 2 stroke making good power, if you are running hard and have a lean mixture for a short time it can easily trash your motor. For the most part a 4 stroke does not care and just loses power. A 2 stroke may make a little more power running lean, until it blows up a short time later. Very high risk to have poor fuel/fuel supply for a 2 stroke. Maybe a low comp/low power weedeater will not care, but most OBs are somewhat stressed for higher output.

[2springers26]

Mercury & polaris... both agree on fuel lasting 30 DAYS..... Not from when you bought it.. but from it's birth... buy it often & from a place that sells alot of it.... the big six us less alchohol.. use the there fuels In 2 strokes...(citco,mobil, standard( bp), shell,66, 76,)

[sho305]

By that last point I mean old fuel has increased chances to clog/gum something up, and lean out your motor...mostly after 6 months in my experience and certainly after a year, but maybe even only 3 months in certain metal carbs. So in my typical 6 mo winter stuff will start ok on the premuim, but I don't use the motor for much until fresh fuel is in there as the octane has gone away and it could detonate. So I am sure at a year old your motor has a good chance of acting funny on that fuel just trying to start it.

So I agree with the other guys, a month old fuel should be fine. Even up to three months it should be ok, but will not be great. So at three months I would not run it in a hipo outboard without putting a large % of fresh higher grade fuel mixed into it. Heck, my force might not care, but I don't even want to take a chance with that and bad fuel since I been running it 300rpm over the top rpm...:) Even with that relatively worthless motor the cost is too high. Then again I have 10 things here I can use the gas in...

I go by performance mostly; so I'll run anything that will run in lawn equipment, but the more power it makes per displacement then the more I am picky with it (especially a 2 stroke). Most newer cars I can tell as the computer retards timing for detonation and you lose power, but in that case it will not hurt it like it would a 2 stroke.

[Markus]

What has happened is that some of the light hydrocarbons have evaporated from that year-old fuel. To start an engine, you need them.

Whether this has led to significantly lower octane (i.e higher risk of knock) or not depends on the fuel blend. If it is a winter gasoline, it definitely has. If it is a summer gasoline, it may or may not have.

To learn more about gas, read the gasoline FAQ. There is a link to it from my links page (see below this post)

[Fish]

Markus,

thanks for the links page, the FAQ on gas is great. Answered all my questions.

Thanks again,

Fish

[sho305]

There is something with the carbs, perhaps the air vent to the bowl does make a difference how fast it evaporates. However some carbs can turn fuel into gunk or even crystals in a short time (seems like cheaper small engine ones). At a lawn equipment distributor I was at for a while, we had mowers with 4 stroke 14hp kawasaki motors that were the worst. They would not start after 2 months no way, but if you drained the carb no problem. Normally a good pull starting motor, but pulled hard. Maybe they had a big air vent. These mowers were in a closed warehouse and had been driven in there, the air vent thing would make sense.

I have a 95 car that lost power at a year old being stored, I could feel the timing get pulled back a lot. At close to two years it would run so poorly I had to put additives in the gas (lots of injector cleaner and a little premium). I let it run for a while to burn it up and put fresh in. It takes regular, a normal four cylinder. I run it a little every month. Of course that efi is not vented only via emissions.

[merklebob]

6 month old fuel 89 pump rated drop to 55[as tested] in 6 months= big repair bill

[us1]

I dump out any fuel older than two months.

[78Vector]

If you want fuel that lasts & is stable use 100LL (115 pump octane) aviation fuel. Aviation fuel has standards that it must not degrade if it stays stored. Many aircraft sit for several months before the wing tanks are emptied. Would'nt be very good to take-off and have an aircraft engine coughing, sputtering and pinging, now would it ?
I run the stuff in my Merc 115 all the time. It is way overkill in octane for my low-compression head, but it can't hurt anything.