Why Small Engines Hate Old Gas — And What to Do About It
The most common cause of small engine failure we diagnose — across lawn mowers, generators, snowblowers, and ATVs — is ethanol-contaminated or varnished fuel. It is preventable and it is expensive when ignored.
The Ethanol Problem
Most gasoline sold in Minnesota contains 10% ethanol (E10). Ethanol absorbs water from the air — which is fine in a car that burns through a tank in a week, but very bad in a lawn mower that may sit with the same tank of fuel for months. As ethanol absorbs water, it separates from the gasoline (called phase separation), sinks to the bottom of the tank, and gets drawn into the carburetor as a water-alcohol mixture that will not combust.
Varnish: What Happens to Unused Fuel
Even without phase separation, the lighter fractions of gasoline evaporate over time. What remains is heavier, more viscous, and — when fully evaporated — leaves a sticky brown or tan varnish residue. This varnish coats the tiny jets and passages inside the carburetor. A passage as small as 0.3mm in the idle jet can be completely blocked by a thin film of varnish, turning a running engine into a problem that requires carburetor removal and cleaning or replacement.
How Quickly Does Fuel Go Bad?
In Minnesota's humidity and temperature swings, untreated E10 gasoline can degrade enough to cause problems in as little as 30 days in a small engine tank. Fuel with stabilizer added before storage can last 12 months or more. The difference between a $5 bottle of stabilizer used correctly and a $150 carburetor cleaning is significant. We see this cost difference play out dozens of times each spring.
What to Do
Use non-ethanol gasoline (E0) when available — many marinas and some gas stations sell it. If E10 is all that is available, add a fuel stabilizer at every fill-up for equipment that will sit more than a few weeks. At end of season, either treat the fuel or drain it completely. Pre-mixed fuel in sealed cans (like TruFuel or VP Power) has a 2+ year shelf life and is worth the extra cost for equipment that starts infrequently.
Related: Small Engine Repair · Carburetor & Fuel System Service · Small Engine Fuel-System Cleaning · ATV Has Old Gas and Won't Run