Old Fuel Can Be Hard on Mowers

Nothing says summer like neighborhoods coming alive with the sounds of lawn mowers and the smell of fresh-cut grass. But getting the lawn mower out of the shed and running properly can be a struggle early in the season. You turn the key and the riding lawn mower roars to life. Then the engine promptly sputters and dies. Before tearing apart the engine or calling a mechanic, look inside the gas tank.

“Every spring, we see issues with mowers caused by homeowners using gasoline left over from the year before,” says Dwight Grosz, a small engine mechanic near Bismarck, N.D. “Over time, untreated fuel begins to break down, which leads to hard starting, poor performance or an engine that won’t start at all.”

Why gasoline goes bad                                                               

What causes gasoline to break down? The first thing to go is gasoline’s volatility. The lightest chemicals evaporate first, leaving a heavier gasoline that doesn’t combust properly. The engine will probably still run, just not as well. 

A more serious problem is oxidization, which is caused by hydrocarbons in the fuel reacting with oxygen to produce new compounds. This results in gum, which can clog gas lines and filters and create deposits in the fuel system. Gummed-up carburetors can be expensive to fix and may not run properly until deposits are removed.

Water that finds its way into fuel tanks through condensation caused by fluctuating temperatures can also cause issues.

How to fix it

If your mower won’t start because you’ve been using old gasoline, you’ll need to remove the old fuel and any built-up residue in the engine. Begin by referring to the owner’s manual for service procedures.

Next, siphon out the old gasoline into a container for proper disposal. Then, if the lawn mower runs for a few seconds and dies, the carburetor might be clogged or have old fuel in the float bowl.

“When the volatile ingredients in fuel evaporate, it leaves a sticky, varnish-like substance that clogs the small jets in carburetors,” says Grosz. “Once that happens, the only solution is to use a carburetor cleaner to remove varnish deposits.”

After cleaning the carburetor, add fresh fuel and a fuel stabilizer to help keep the system clean.

After treating the fuel

Next, go through a quick checklist to ensure your mower’s ready for the season. Consult your owner’s manual for maintenance recommendations.

Change the oil to remove contaminants, sludge and acids. Drain the old oil out and refill the crankcase with oil manufactured specifically for smaller engines and lawn mowers, such as Cenex® 2-Cycle Oil. Consult your owner’s manual for manufacturer recommendations.

Next, replace the air filter. Last, don’t forget to sharpen the mower blade and remove any grass that’s caked to the underside of the motor deck.

How to avoid future issues

To avoid future issues with stale fuel, try not to store gasoline in tanks or containers for more than two months. If you know gasoline will be sitting for longer than that, add a fuel stabilizer. This will help prevent oxidization.

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