Fuel: Fuel tank is empty or shutoff valve is closed, or fuel line or fuel tank cap vent is clogged. Old gas or water in the gas tank.
Carburetor is overchoked, improperly adjusted or needs service.
Air filter is dirty, or too damaged, it might be better to just replace it.
Spark plug is fouled, improperly gapped, or damaged. Make sure your spark plugs are tight, clean, and connected securely. Nearly every small engine maintenance book will recommend changing the spark plug every season.
Ignition coil or its wiring is defective or ignition switch is off, or timing way off (e.g., broken flywheel key).
Carburetor is dirty, clogged or overchoked, and needs cleaning or replacement.
Gasoline is dirty or stale, and needs replacement.
Spark plug is dirty or defective and needs replacement.
Fuel filter, fuel line, or fuel tank is blocked.
Carburetor is dirty.
Air leak in carburetor or intake manifold (e.g., bad O-ring, gasket, primer).
Carburetor is improperly adjusted or needs service.
Magneto is not timed properly.
Carburetor is set too lean.
Engine has overheated.
Carbon buildupin combustion chamber.
Flywheel is loose.
Connecting rod is loose or worn.
Cylinder is excessively worn.