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Engine won't start

Fuel line

Is the fuel fresh?

If the fuel is over 30 days old, replace with fresh, treated fuel. (see Why does my engine start and then shut down? and Engine does not start/runs poorly after storage).

Is the fuel tank empty?

Fill the fuel tank; if the engine is still hot, wait until it has cooled before filling the tank (see Removing and Cleaning the Fuel Tank).

Is the shut-off valve closed?

Open the fuel shut-off valve (see Removing and Cleaning the Fuel Tank).

Is the fuel diluted with water?

Empty the tank, replace the fuel and check for leaks in the fuel tank cap (see Removing and Cleaning the Fuel Tank).

Is the fuel line or inlet screen blocked?

Disconnect the inlet screen from the engine and clean it, using compressed air. Do not use compressed air near the engine (see Removing and Cleaning the Fuel Tank).

Is the fuel tank cap clogged or un-vented?

Make sure the cap is vented and that air holes are not clogged (see Removing and Cleaning the Fuel Tank).

Carburetor

Is the carburetor blocked?

Remove the spark plug lead and spark plug; pour a teaspoon of fuel directly into the cylinder; reinsert the spark plug and lead; start the engine; if it runs for a moment before quitting, overhaul the carburetor (see Adjusting the Carburetor).

Is the engine flooded?

Adjust the float in the fuel bowl, if adjustable; make sure the choke is not set too high (see Adjusting the Carburetor).

Is the fuel solenoid (if equipped) functioning properly?

Test the solenoid (if equipped) to ensure it is functioning properly (see Inspecting the anti-afterfire solenoid).

Ignition

Is the spark plug fouled?

Remove the spark plug; clean the contacts or replace the plug (see Servicing Spark Plugs).

Is the spark plug gap set incorrectly?

Remove the spark plug; reset the gap (see Servicing Spark Plugs).

Is the spark plug lead faulty?

Test the lead with a spark tester, then test the engine (see Servicing Spark Plugs).

Is the kill switch shorted?

Repair or replace the kill switch (see Servicing the Brake).

Is the flywheel key damaged? Did the engine stop all-of-the-sudden after striking an object?

Did the engine abruptly stop (ex. blade strike an object)? See if the flywheel key is sheared. If so, replace the flywheel key, re-torque the flywheel nut to proper specifications, then try to start the engine; if it still won't start, check the ignition armature, wire connections or, in some engines, the points (see Replacing the Ignition).

Compression

Are the valves, piston, cylinder or connecting rod damaged?

Perform a compression test (see Additional Maintenance) if the test indicates poor compression, inspect the valves, piston and cylinder for damage and repair them as needed (see Removing Carbon Deposits) and Servicing the Valves).