Well, it's one of 4 things...mechanical, electrical, fuel or air. 
  1) Do you have compression - could be a valve problem
  2) Is fuel getting to the spark plug - you could try removing the spark plug and put about a tablespoon of gas into the engine and the replace the spark plug ( reattaching ignition wire ) and see if it starts - might run for about 5 seconds.
  3) Check the air filter - try removing it and see if it starts.
  4) You could have carb problems - Remove the air cleaner and try spraying some carb cleaner into the carb. You may need to put in a carb kit. 
  5) Does the carb have floats ? If so, then that could be your prob.
    Hope this helps  - best I can do with limited info.
                
 Kevin K.   Woodbury  MN.
                
 August 2005