Grab a can of carb cleaner & a pair of red in line spark testers.
Fit the spark testers then go mow.
When you shut down the engine & it won't start again , crank the engine and look at the testers.
If they are still flashin bright red then the ignition is not you problem.
Now give the carb short shot of carb cleaner.
If the engine fires then you have a carb / fuel delivery problem
No joy the pull a plug and give the cylinder a SHORT shot of cleaner the refit the plug and crank again.
do both sides , one at a time.
If it fires now but not when sprayed down the carb, you have a valve problem.