The part(s) or condition(s) listed below for the symptom Small engine won’t start are ordered from most likely to least likely to occur. Check or test each item, starting with the items at the top of the page.
Inspect the spark plug for signs of wear or damage. If the porcelain insulator is cracked, an electrode is burned away or damaged, or there is heavy carbon buildup at the electrode, replace the spark plug. To determine if the spark plug is defective, use a spark plug tester. You should see a strong spark between the tester’s terminals when the engine is cranking. If there is no spark, this indicates that the spark plug is defective and should be replaced.
The carburetor might be clogged. A clogged carburetor is most commonly caused by leaving fuel in the engine for a long period of time. Over time, some of the ingredients in the fuel may evaporate, leaving behind a thicker, stickier substance. This sticky fuel can clog up the carburetor and prevent the engine from starting. If the carburetor is clogged, try cleaning it with carburetor cleaner. If cleaning the carburetor isn’t effective, rebuild or replace the entire carburetor.
The ignition coil sends voltage to the spark plug while the engine is running. If the ignition coil is defective, the engine may not start. Before replacing the ignition coil, ensure that the spark plug is working properly. If you have confirmed that the spark plug is working properly, test the ignition coil with an ignition coil tester. If the ignition coil is defective, replace it.
The flywheel key might have sheared in half. The flywheel key is a small metal piece which fits into the crankshaft and engages with the flywheel. If the engine stops suddenly, the flywheel key breaks in half to prevent damage to the engine. To determine if the flywheel key is broken, remove the flywheel from the engine and inspect the flywheel key. If the flywheel key is broken, replace it.