Does Old Gas Cause a Mower to Not Start?

Shelf Life

• Gasoline that has been in storage for a month or more begins to deteriorate. Acid levels rise, and varnish and gum can form in the fuel system components. Even a light varnish is enough to obstruct the minute passages in the carburetor and block flow through a fuel filter. Restricted or clogged fuel delivery can prevent the engine from starting, and tainted fuel lacks the volatile characteristics needed for combustion.

Fuel Cleaners

• Adding a carburetor and choke cleaner to stale fuel may help dissolve minor deposits. The cleaner can be added to the fuel tank, along with fresh fuel. Then the engine is turned over a few times to distribute the cleaner throughout the fuel system and allowed some time to work through any varnish or gum. Proper function may be restored in as little as an hour, and the remaining cleaner will continue to help keep the gum out.

Major Pains

• Severe cases of fuel gumming and varnish may require more extreme methods of correction, including fuel system component replacement or repair. A contaminated fuel filter must be replaced to prevent any downstream spread of major impurities. Stale fuel should be drained from the tank and carburetor. Some carburetors are fitted with a drain plug to aid the evacuation process, but a gummed-up carburetor may need to be taken apart for cleaning or overhaul.

Stable Fuel

• Gasoline shelf life can be significantly extended with the use of fuel stabilizers. A stabilizer has elements that prevent impurities from forming in the engine and can be added to fresh fuel at the end of the mowing season. Fuel tank filler cap vents can also be sealed off while in storage, thus preventing evaporation. Evaporation can condense the fuel and initiate gum formation. Condensation in the tank also accelerates the formation of harmful acids.