It has happened to us all. The engine in your bike / car / truck / boat / lawnmower / etc. was running just fine and all the sudden it is missing, coughing, sputtering, shuddering, stalling out, and maybe even now it won't start, even though you have lots of gas and the engine spins up normally when you hit the starter...it just won't start or runs like crap. What the heck?
To be fair, there are LOTS of reasons a properly running engine can suddenly go bad, but this posting is about one of the most common reasons: Bad Gas...or more accurately, water in the gas tank.
So when are you justified in thinking the problem may be bad gas when your engine suddenly starts to run badly?
Why is water in your gas a problem? Gasoline burns. Water? Not so much. A little water in your gas can make the engine run badly. A big slug of water getting into your fuel system can bring your engine to a sudden and ignoble stop.
But how does water get into your gas tank? There are lots of possible ways. The ugly ones all revolve around the fuel having been handled badly before it got to you. Perhaps a storage tank had a leak or a loose lid that let rainwater get in. But that sort of thing is very rare. The most common way for water to get into gas tanks is for it to come in as humid air.
Air can hold more water when it is warm than when it is cold. That is why the weather report always talks about Relative Humidity. The RH is tells us how much water vapor is dissolved into the air relative to how much it can hold at that specific temperature. So if if an air sample is at 50% RH at a specific temperature, it will have a higher RH at a lower temp and a lower RH at a higher temperature. The actual amount of water in the air sample will not have changed. What changes with temperature is the air's ability to hold that water vapor in solution. And if you cool it enough, you eventually reach the Dew Point for that air sample, the temperature at which the water vapor starts to condense out of the air, i.e. the water vapor begins to turn into liquid water.
If you capture a dry jar full of air outdoors on a warm day, close it up tight, bring it inside and stash it in your refrigerator, when you come back the next day (after the air has gotten nice and cold inside the jar) you will find some liquid water inside the jar. As the air cooled it could no longer hold the water vapor, and the water began to condense (water vapor converted back to liquid water). Whatever the relative humidity of the captured air might have been outside, as the air cooled, the relative humidity inside the jar got higher and higher until it got to about 100% (meaning it became as humid as it could be before the water vapor condensed into liquid water in the jar). The more water vapor that was dissolved into the air sample to start with (the higher the RH was when you captured the air), the easier it will be to get the water to condense out as the temperature of the sample falls...and the more water will end up in your jar.
This all matters because most fuel handling systems, at least briefly, allow air to get in now and then. Once air is in a storage tank, gas can, or fuel tank, it becomes possible for the moisture in that air to condense into your gas tank. So why are big weather changes harbingers of bad gas? The easy one is a cold snap. Hot humid air got into the tank back when it was warm outside, then a cold night comes around and the humid air in the tank condenses into water as the tank cools. But it is also possible to have a fuel storage tank that has been cold that gets exposed suddenly warmer and more humid air. The warm humid air gets into a cold tank and, bingo, the cold fuel chills the humid air and once again there is water in the tank. Generally, the onset of winter is more likely to cause problems, but springtime can also mess with our fuel supply.
Still, why is a little water in a gas tank a problem? What harm can a little water do when mixed with gallons of gasoline? The problem is that they do not mix. Water sinks to the bottom of a tank filled with gas or oil. Here's a sample of some contaminated fuel. The line of separation is the natural result of trying to mix gas and water. The water is the bottom layer.
Once water is in the tank, it tends to go to the bottom, which of course is where the fuel pick-up is located (so you can get the last little bit of gas out of your tank as you get near empty). Now, the sample shown above is a badly contaminated sample. Your tank may only have a tablespoon full of water in it. And it may be sloshing around as your vehicle moves so that the little slug of water only rolls by the fuel pick-up now and then, and only briefly at that. That can be enough to make your engine shudder, buck, and cough...and if you are unlucky enough, the fuel pump may suck up enough at one time to stall your engine.
So, what to do? We could try to dump your gas tank or suck out the water, but both are usually impractical. Fortunately, there is a better option: Mix the water with something that will burn. Gasoline is not the only flammable liquid in the world. Some liquids, like alcohol, for example, mix beautifully with water and are also flammable. But the liquid I prefer to use is Berryman's B-12.
The Berryman company does not pay me a nickel to say nice things about their stuff. This is just the straight skinny. B-12 is GREAT. It is a super fuel system cleaner, but it is a spectacular water dispersant. I carry a can of B-12 with me in each of my vehicles...because you never know when you are going to get a tank of bad gas...and if you do, B-12 is the almost instant cure. And when you can buy it for less than $5 a can, it is absolutely the cheapest miracle cure available on the planet. And yes, I run a can through each of my vehicles once or twice a year just for good measure.
Notice that you are seeing a can of B-12 LIQUID. They also sell B-12 in spray form for spot cleaning fuel system components. It is great, but get the liquid for dealing with bad gas.
So, when the day comes that you get the dreaded sputter / cough / stall routine from your engine after a fillup, dump a whole can of B-12 into your gas tank. Give it a few minutes to mix with whatever is in your tank, and then try to go again. In a car, the tank is usually in the back and the engine is usually in the front, so the pipe that connects them can be a bit long. Don't be too dismayed if it takes several attempts to get the improved fuel up to the engine. As a rule, fuel does not flow unless the engine is turning (turning, even if not necessarily running). The moral to that story, by the way, is if you notice your engine starting to run rough after a fill-up, don't wait until it dies to put in the B-12. Put it in right away. The problem will clear much more quickly and easily if the engine is running, even running badly, than if it is not running at all. And then go get another can of B-12 at an auto parts store as soon as you can and stash it in your trunk for the next time you get bad gas.
So that's it. Generally, jumping to conclusions when something goes wrong is a bad idea. As a rule it is much better to figure out what has gone wrong before attempting a cure. But in this case, a can of B-12 is so cheap, and putting it into your tank is so easy, even if it fails to fix your problem, you have lost very little time and money. So if you have the least suspicion you are a victim of bad gas, go for it. It can't hurt and it may well be the cure.