Re: lawn tractor
Don't forget about the low oil sensor. If the motor cranks over and no start - pull the plug to check for spark,,,,,If you got spark - open the drain plug on the bowl of the carb. The float maybe stuck preventing fresh gas entering the carb. Sometimes tapping on the bowl is enough to dislodge the needle float valve. If not, remove the bowl (take care not to damage the gasket. Pull down on the float if it is stuck in the up position for shutting the fuel off. If you have gas and spark, check the oil level and or bypass the sensor to determine if it was it. If it still doesn't start, hold you thumb/finger over the spark plug hole (make sure you don't come in contact with the plug wire) ,,,,it should blow some compression against your finger. If it has lost its compression,,,could be rings stuck or a valve stuck that is not closing.
If it doesn't crank,,,then start checking all the safeties such as the seat, blade, shifter, clutch/brake, and even the switch that is surpose to disengage the blade when put into reverse.
Good Luck,,,,,,,,,,SS