What Causes a Mower to Miss Out When Hot?

How an Internal Combustion Engine Works

• A four-stroke internal combustion engine uses two downward strokes and two upward strokes of a piston to complete a combustion cycle. The first downward stroke draws in a mixture of air and fuel through the intake valve. The valve closes, and the first upward stroke compresses the fuel/air mixture; this mixture is ignited at the top of the stroke. The ignition of the mixture causes a controlled explosion that forces the piston downward; this stroke is called the “power” stroke. Finally, the burned gases are exhausted from the combustion chamber and through the exhaust valve by the second upward stroke.

How Elevated Temperatures May Affect Fuel Systems

• The fuel in a fuel/air mixture must enter the combustion chamber as a mist. Elevated temperatures or operation at high altitudes can cause fuel to vaporize before the fuel reaches the engine; this condition is often called “vapor lock.” Vaporized fuel in a fuel line causes the fuel pressure to drop and may prevent fuel from reaching the combustion chamber. This may cause the engine to “miss” or even stall completely. A hot engine may contribute to fuel vaporization if the fuel pump or the line from the fuel tank is close to the engine.

Other Factors May Affect Combustion

• Much of the gasoline sold in the United States contains ethanol. Ethanol has a lower evaporation point than does gasoline and can absorb water. These two factors make gasoline that has been stored for more than a month more prone to causing a vapor lock or an engine that is “missing out” in warmer climates or in high-altitude areas than fresh gasoline.

Preventing Fuel-related Problems During Warm Weather

• Vapor locking can be prevented or minimized by using only fresh fuel when you plan to use a lawn mower on a warm day. If the lawn mower engine begins to “miss” or stall after the engine gets hot, turn off the engine and allow the engine to cool down before using the lawn mower again. If the lawn mower has consistent problems with stalling or “missing” when the engine gets hot, you may also wish to have a small-engine mechanic inspect the fuel delivery and carburetor for problems.