Sound as if there might be too much fuel and not enough air. If air filter clogged up suggest that you clean it, or just to test and see if you can start engine, remove filter and try to start without filter- this will allow more air .
Have you checked the spark. Take spark plug out and rest it on its side on the clylinder head, pull the start cord and see if there is a spark between centre and side electrode.
When you remove the spark plug look at the end see if it is damp with petrol - if it is may be as I suggested too much fuel insufficient air. Clean plug and replace try starting
Maurice
August 2010