Generators and boat motors.
They sit, a lot. Fuel not only varnishes, but before it even does that, evaporation causes issues. The "good" parts of gasoline evaporate out and leave the junk behind, and this happens kinda slowly, starting at around 2 weeks from the time it gets pumped into your fuel can from the gas station. Yes 2 weeks. Less in the summer months.
Here is something else I learned about fuel. Winter vs summer. There is summer blended fuel and winter blended fuel. Winter blend is formulated to evaporate a little better when run through a car engine, so that the engine warms up quicker. But the downside is that if you have any left over, pour it into the generator (or atv, whatever)-and the air temp is higher than, as low as 80 degrees, the fuel can actually boil. I have a video of boiling fuel in a 6 gallon portable fuel tank, took it a couple years ago when a guy brought in his boat because it wouldn't run right...turned out to be winter fuel. Soon as I pulled the cap off, you could literally watch it boil. It happens. When it's boiling, some of it is evaporating. Gasoline molecules are tiny and they can escape right through fuel lines if they're not EPA certified low-permeation lines. Notice I said "LOW" permeation-to my knowledge no line is "no permeation". It runs through the rubber lines slow enough that you never smell it either. Many don't understand this, but that's what happens. Summer blended fuel lasts a little longer, but the 2 week rule should still be applied.
something else that many don't consider is application. Gasoline is designed specifically for CARS, and all cars (well 99.99% of them) are fuel injected, which EFI cars don't have a need for a higher boiling point. So when gas is used in a small engine, that small engine may or may not run quite right, and it seems to be worse with older engines that have old style carburetors. Remember the old Briggs flo-jet carbs? The carb was bolted directly to the gas tank, then the muffler was an inch away from it. They'll still work today but not optimally obviously.
octane. THe best octane fuel is the lowest octane that your engine will run on without detonation. A 6:1 briggs will be happy with 87 octane. Heck it'd probably be happy with 76 octane. Running "the good stuff" is a waste of money most of the time. It also has a tendency to carbon foul the combustion chambers, piston tops, and piston rings quite a bit more than the cheap stuff does. And another thing about high octane. "Most" cars and trucks are designed to run on 87 octane with up to 10% ethanol (E-10) without issue. That means that if a gas station has 3 10,000 gallon tanks underground with 87 in one, 89 in another, and 93 in the other tank, the 87 octane tank will be bone dry long before the 89 and 93. So the "good stuff" sits in the tanks longer-and is more likely to go sour and/or develop condensate moisture. Another reason to stick with the cheap stuff.
Ethanol. Most engines since the 1990's will be fine with E-10. Non-ethanol fuels typically suffer the same problems as 89 and 93 octane...in that they sit in the tanks longer, and can go bad just sitting in those tanks at the gas station. So IMO, run the up to 10% lower octane, UNLESS the equipment specifies a higher octane. Only ones I can think of are some higher performance ATV's, motorcycles, and some cars. Ethanol in itself has not caused any problems in my business-it's the fuel mix itself going bad....the Ethanol in itself cannot be blamed, although people can and certainly DO blame it entirely. I don't care what X magazine or internet article says about Ethanol being "bad" for small engines....it simply is not the entire blame, it's the changes in overall fuel quality that are to blame. This based on doing thousands of fuel system repairs every year, not over analyzing anything-just customer feedback and personal experience.
Also on the Ethanol stuff, most governing bodies require fuel to be labeled "up to 10% ethanol" if the Ethanol content is 7% minimum. But what if it's only 5%? Can it be labeled and sold as "non ethanol"? If so, and gas stations are charging up to $.50 more per gal for non ethanol, then who's getting the shaft? I have test strips at work specifically for testing but the company doesn't pay for them and in most cases I don't even use them except for that one customer that doesn't listen (and it happens). Even then it's got to be fresh fuel that you're testing as the evaporation effect can change things.
2 weeks. Just remember the 2 week rule and you'll be fine. If the fuel is going to sit for more than 2 weeks, remove it from the equipment and dump it into something that'll use it up, and you won't have a problem. Is it a pain in the back side? Yup. But it's a lot less of a pain than, in this case, having no backup generator when you need it.
