Last fall my leaf blower started acting up in the middle of a job. It would run for a minute, then sputter and die. Usual culprit, I figured: gas that had been sitting in the can since August. I drained it, put in fresh fuel, and still got the same behavior. A friend who restores old motorcycles told me to look closer at what happens when ethanol gas sits even for a few weeks in a plastic tank. That got me down a rabbit hole I didn’t expect, and it changed how I buy fuel additives.
What I found is that the ethanol in pump gas does something nasty called phase separation. You don’t have to let gas sit for months to see it. Even a few days in humid air can cause water to bond with the ethanol and fall out of the gasoline. That watery sludge sinks to the bottom of your fuel system and finds its way into the carburetor jets. Most small engines don’t have a water separator. Once that happens, you are cleaning the carb or replacing it. So the real problem isn’t old gas. It’s wet gas. And the best fix I have found is treating every gallon I buy with a stabilizer that handles ethanol, not just storage stabilizer. I started reading up on stabilizers and eventually ordered from a place called sta bil shop that specializes in the original formula. That made a difference I could hear on the first idle.
How ethanol phase separation works
Gasoline itself is a hydrocarbon blend that does not absorb water. Ethanol, on the other hand, pulls moisture out of the air like a sponge. Pump gas in the US typically holds ten percent ethanol. When the ethanol absorbs enough water, the mixture exceeds its tolerance. The water-loaded ethanol then separates from the gasoline and sinks to the bottom. That is phase separation. Straight gas would separate too if it contained water, but ethanol makes it happen faster because it actively attracts water. I have seen this happen in less than a week in a half-full gas can during humid summer weather. The separated layer is a corrosive mix that gums up carburetors and damages rubber seals. Many people blame old gas for engine problems, but the real cause is that phase separation happens long before the gas loses its combustibility. If you store your mower or trimer with untreated ethanol gas, you are likely to have this issue even if you used it last week.
What I do now to keep engines running
I have moved to treating every gallon of gas I buy with a fuel stabilizer that specifically says it handles ethanol phase separation. The regular blue stuff at the hardware store is fine for winter storage but does not stop water absorption in the short term. The original Sta-Bil formula with the red label is what I use. I buy it in the concentrated kind so I can treat a few gallons at a time. I also stopped filling gas cans all the way. A full can leaves less air space for humidity. I keep my gas cans in the shade and use them within a month. For my mower and generator, I run the carb dry before I let them sit more than two weeks. It sounds like extra work, but it saves me the cost of carb kits and the frustration of a machine that refuses to start. I also change to non-ethanol gas for my chainsaw and string trimmer when I can find it, but that is not available everywhere. In those cases the stabilizer is the only defense. I do not claim this solves every small engine problem, but it has made mine reliable enough that I only need to clean a carb now when I let my guard down. And I think paying attention to phase separation is more important than most people realize. It might be why your engine dies mid-job with fresh gas. It was for me.