A lawnmower engine, as it ages, will start showing many problems and symptoms. However, especially in older engines, a single problem in one area may not be the only problem, just another symptom of a larger problem. For this reason, thoroughly check all potential areas on the mower for worn or malfunctioning parts.
Check Fuel Supply
Fuel problems cause most engine problems on a lawnmower. Check that the fuel tank is topped off with fresh fuel, no older than 30 days. If you used old fuel or it was left in the tank for more than 30 days, perform a thorough cleaning of the fuel system. Clean out the gas tank and air filter. Pull out the fuel filter if present and replace the filter. If the fuel lines haven't been replaced this season, replace both fuel lines and make sure they are properly connected to the carburetor.
Check Spark Plug
The spark plug ignites the fuel in the cylinder. If the spark is too weak, the fuel will combust poorly. Pull the spark plug from the cylinder block and replace it if it looks old or is covered in carbon; don't try to clean and re-gap the old plug. Make sure the ignition switch is engaged and the transmission, if necessary, is in neutral. Use an ignition module tester to check the function of the ignition module, plug wires and spark plug. If no spark is present during the test, replace the ignition module or spark plug wires.
Check Oil Supply
A lawnmower engine needs a constant supply of lubrication to keep the piston from grinding on the cylinder walls. The oil also acts as a coolant for the crankcase and cylinder. When the oil level drops too low, most lawnmower engines will automatically shut off to prevent further damage to the cylinder and crankcase. Pull out the oil dipstick and check the level with a towel or rag. If the oil reading is low, fill the engine's oil tank until it reads full. Change the lawnmower's oil every 25 to 30 hours of operation.
A Dirty Carburetor
The carburetor brings fuel into the engine, measures the amount needed for the engine speed and mixes it with air. After several years, the fuel will get the inside of the carburetor dirty, preventing the carburetor from doing its job. With a dirty carburetor, the engine will get starved of fuel and shut off. With so many different styles of carburetors on the market, most lawnmower manufacturers recommend taking the mower to a mechanic for all carburetor repairs and adjustments.
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