Posted By RichC on March 10, 2015
When regular leaded gasoline gets old, a thin layer of varnish coats the inside of fuel lines and the jets of the carburetor. This can plug or narrow jets that meter fuel into the carburetor and with cause the engine to either run lean or not run (start) at all. For fuel efficient and environment approved modern engines, this make them even run leaner because the jets are smaller than they should be. This creates an ultra-lean condition and can make an engine run hot which can damage the pistons, score the cylinder walls and shorten engine life. If you are running leaded fuel, a quick fix is a can of fuel system cleaner as this will dissolve the varnish in most cases (ie. winter storage).
For most of us though, we are running ethanol based gasoline. This additive doesn’t produce varnish but makes a sticky gum that clogs up jets even more stubbornly than varnish. Ethanol doesn't have to sit long before it begins to gum up small jets. It was the most likely what caused my Poulan chainsaw to run lean and hot … damaging the piston and cylinder walls. With lawnmowers and outboard motors (stored seasonally) these are the most common non-starting problems. This can usually be prevented by adding a fuel stabilizer or purging the alcohol based before storing an engine.
According to what I've read, not all fuel stabilizers are the same. Some are designed to reduce varnish in leaded fuels and are not effective with ethanol fuels. One recommended stabilizer that I can recommend is Starbrite's Star-Tron Enzyme Fuel Treatment. (not a paid endorsment)