is the gas shut off valve open? fuel filter plugged up? kinks in the fuel line? float in the carb stuck?
check for spark first...you should have a constant, strong spark at the end of the spark plug when you do what al said to check for spark.
if you have good, strong constant spark, then you more than likely have a fuel problem or a REALLY bad air leak...possibly a bad head gasket.
is this engine brand new, or has it been ran before?
check your wiring connections and get rid of the snap together wire connectors. solder all wire connections and cover them with heat shrink or black electrical tape.
make sure the white wire coming from the engine is not touching anything and is cut off clean and covered with heat shrink or black electrical tape. make sure no wires have rubbed through or broken insulation. make sure no bare wires are touching the frame or the engine.
if you have anything hooked to the white wire, disconnect it and try to run the engine.