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First you should have the battery tested as a weak battery can be causing this. If the battery is fully charged and the problem is still happening the engine may just be flooded. Try holding your foot on the gas and crank the engine. Do this with a jumper box installed on the battery as this will wear down the battery quickly. Also do this 20 seconds at a time taking a minute break to cool down the stater. When that happens and the engine still will not start when holding your foot on the gas then the spark plugs will need to be removed to clean or replace them. Once removed squirt a few drops of oil down into the cylinders to build back compression. If the clear flood does not work we will need to check for fuel pressure, spark and compression.