Lawn Mower Problems

Engine will not start:
Make sure that the ignition switch is on.
All attachments are disengaged.
Transmission is in neutral.
Most machines have safety mechanisms that will not allow them to start. Owner's manual will have starting instructions for your mower.

Check for spark.
Remove the spark plug and "ground" hex part to bare metal.
Remove any spilled gas.
Spin engine by pulling rope or turning key.
A blue spark should be visible.

No spark? Spark is yellow or weak? Try new spark plug.
Still no spark?
On older machines try cleaning and setting the points.
On newer machines replace the electronic ignition module.
Test old module before replacing it: disconnect all wires from module terminal that connects to the kill switches and check again for spark. If a spark is visible then a wire or switch is defective.
Also check the flywheel key which affects spark timing. The key may need replacing if machine strikes object while mowing and the engine is stopped.

More here: m-and-d.com/helpfaq.html --- includes:
Check the carb & fuel system
Engine will start but runs poorly or dies
Engine starts and dies, surges, or will only run on choke
Carburetor problems produce different symptoms.
Bowl-type carbs, Runs then dies.
Surging, only runs on choke, sputtering.
Is the air filter dirty?
Diaphragm type carbs.
Miscellaneous engine troubleshooting
Oil smoke or leak near the carb
Oil leak near the base of the engine
Oil smoke thru the muffler
Backfires thru carb or air filter
Kicks back when starting
Starter rope will not retract or is hard to pull
Starter rope broke
Compression low
Won't start when hot
Battery charging troubleshooting
If the battery will not hold a charge
If the engine cranks too slowly
If the starter spins, but doesn't engage the engine
Electric starter troubleshooting
Test the solenoid
Test the starter switch