Small engine repair covers the diagnosis and servicing of engines found in lawn mowers, chainsaws, generators, tillers, water pumps, and other homestead equipment. Most small engines are single-cylinder, air-cooled, four-stroke gasoline engines ranging from 2 to 25 horsepower. Two-stroke engines are used in chainsaws, string trimmers, and leaf blowers. The three essential requirements for engine operation are compression, fuel-air mixture, and spark. Diagnosis follows a logical sequence of checking these three systems. [[Chainsaw Maintenance]] and [[Generator Maintenance]] address equipment-specific maintenance procedures.
Low compression prevents the engine from running or causes hard starting. Compression is tested with a compression gauge inserted into the spark plug hole. A reading below 60 psi for most small engines indicates ring, cylinder, or valve wear. Valve clearance should be checked and adjusted with a feeler gauge, typically 0.10 to 0.15 millimeters (0.004 to 0.006 inches) for intake and 0.15 to 0.20 millimeters (0.006 to 0.008 inches) for exhaust on most engines. Head gasket failure presents as loss of compression, oil contamination in the cooling fins, or external leakage.
Carburetor problems are the most common cause of small engine failure. Fuel left in the carburetor for more than 30 days evaporates and leaves varnish deposits that clog jets and passages. Cleaning the carburetor involves disassembly, spraying cleaner through all passages, and clearing jets with compressed air. Ethanol-blended fuels cause corrosion of aluminum carburetor bodies and swell rubber diaphragms. Gaskets and diaphragms should be replaced when cleaning. The fuel filter and fuel lines should be replaced if hardened or brittle.
Small engine ignition systems consist of a magneto, ignition coil, spark plug, and kill switch. Spark is tested by grounding the spark plug against the engine block and pulling the starter rope, looking for a bright blue spark. A weak or absent spark indicates a faulty ignition coil, magneto air gap out of specification, or a grounded kill switch. The magneto air gap is typically 0.25 to 0.30 millimeters (0.010 to 0.012 inches). The spark plug should be gapped to the manufacturer's specification, usually 0.60 to 0.80 millimeters (0.025 to 0.030 inches), and replaced annually.