Check the spark plugs could be filled with carbon - need to clean every contact point before you return them.
If it does not work try the high tension cable wires it may have some current leak.. need to change it. Also test the terminals of your battery if it is loose you will not get full start you need to clean them both and put some battery terminal grease to remove some acid build up (white stuff).
check on the air filter if you find it too clogged chances are it does not give anymore air(oxygen) to complete the combustion.
check the fuel supply if there's no fuel there will be no combustion..
you know all these... try it and you will get somewhere.
