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Air, fuel, spark at the right time and compression are required.
1.Fuel cap venting. Hardly ever the cause.
2.Lack of fuel delivery or dirty carb. Trash in the fuel tank outlet normally covers your main complaint.
3.Stripped flywheel key.
4.Water in the gas.
5.Gas can with no spout passing trash, dirt, grit, leaves, bugs and water to the mower tank.
6.Fouled plug/s.
7.Valves way out of spec. As in loose rocker arms and so on.
Most of the time posts like this are number 2 above.