Use a spark tester to make sure you are getting a good strong spark between the plug wire and spark plug. An open spark done with a screw driver should be almost blue in color - not a weak yellow. If is is weak, you have an ignition system problem. If you have the spark, pull a plug after a start attempt and see if it is wet with gas. If it is wet, it could be just fouled plugs. If it is dry, check for 32psi fuel pressure at the fuel rail during start attempt. Low fuel pressure means a fuel supply problem. For an engine to run you have to have fuel, compression, air and in a gas engine...spark.