When straight gasoline gets old, it makes a layer of varnish that coats the inside of everything wet with fuel. After a while, this narrows the annulus through the jets that meter fuel into the carburetor, which makes your engine run lean. And modern engines already run on the leans side of driveability, for emissions-mandated reasons. Subtract a little fuel, because the metering jets are partially plugged, and you’ve got an ultra-lean condition that can make your engine run hot, which makes it run leaner in turn. A highly-tuned engine—like the one in your sports car, chainsaw or motorcycle, can actually run hot enough to scuff a piston, damaging the cylinder walls. Fortunately, if there is a little varnish, a can of fuel system cleaner usually dissolves the varnish and scours the inside of the system back to some semblance of normal overnight. Serious varnish issues may require disassembly of the fuel system and cleaning with corrosive carb cleaner.
Ethanol doesn’t produce varnish. Instead, it makes a sticky gum that clogs up jets even more stubbornly than varnish, and it can produce huge amounts of gum in only a few weeks. I was testing some fuel in a 4500-watt generator with a freshly-rebuilt carburetor last spring, and neglected to add fuel stabilizer to the E10 remaining in the tank after finishing. Not quite three months later, I wanted to loan it to a friend who had a basement with two feet of water and, thanks to Hurricane Irene, no electricity for three days. There was so much gum in the float bowl that I had to use a toothpick to unclog the completely-occluded main jet. A completer rebuild, including flushing the fuel tank three times with lacquer thinner, replacing the fuel hose and filter, and complete carb cleaning in the carburetor caustic bucket got it back up and running. Time lost: about three hours including a trip to the auto parts store for fuel line and a new filter. This could have been avoided if I had added fuel stabilizer.
There are fuel stabilizers, and then there are fuel stabilizers. any is better than none, but old-line stabilizers were intended to reduce varnish, not gum. There are several on the market, bu most seem to be based on the same technology; mineral-spirits based solvents. They’re not as effective as the organic, alcohol-produced gum we’re seeing from ethanol. The one I’ve used for best results is Star-Tron’s Star-Brite. There may be others.
As always, follow the directions on the label.