Why Does My Mower Bog Down When the Engine Gets Hot?

Vapor Lock

• Fuel starts at the tank and travels up the hose to the carburetor. This happens because the pressure inside the carburetor is greater than the pressure in the tank. If the pressure levels in the fuel tank increase above the carburetor's, the engine will start bogging down because of the difference in pressure levels, which is commonly referred to as vapor lock. The gases in the tank need venting to restore pressure. A plugged vent hole on the fuel cap usually causes vapor lock.

Clogged Spark Arrestor

• After the fuel ignites in the combustion chamber, the heated air travels away from the engine via the muffler. Inside the muffler, the spark arrestor screen traps any still ignited particles, preventing them from leaving the engine and starting a fire. Tthis screen also traps all of the carbon deposits, eventually choking off any outgoing air. Spark arrestor screens need a thorough cleaning after every 40 to 60 hours of operation, and replacement if they can't be cleaned.

Low Oil

• The oil in a lawn mower exists primarily to keep the crankcase lubricated so it can keep moving at high rates of speed. The secondary function of the oil is to help keep the engine cool enough to keep operating. If the oil level drops inside the crankcase, it causes the mower to start heating up, and the mower will shut off to prevent frying the engine completely. Before every use check the oil level and fill it, if necessary. Lawn mowers usually need oil changes after every 25 hours.

Clogged Air Filter

• As the engine gets hotter during operation, it needs more incoming air to stay cool enough to keep working. The engine sucks new air in through the air filter and the carburetor. Since the mower works in dusty, dirty conditions, this air filter can get clogged to the point where air stops coming in. This causes a sharp loss in performance, such as a bog down. Air filters need cleaning after every 10 hours of operation.