It could be a blocked carbeurettor, blocked fuel pipes or maybe a dirty spark plug.
if i was you, i would start with the spark plug first, remove it, and turn the engine over a few times to get rid of any excess petrol (could be flooded) then place the spark plug against the side of the engine and leave it plugged in, then turn the engine over again, and if you dont see a spark, then clean it with a wire brush, or replace it.
if this doesnt work, then try unblocking the carbeurettor.
stephan, July 2007