It sounds to me as if it is definitely a fuel problem, follow the instructions below and then re-fit all the carb and air filter.
Remove the carb and throughly clean.
Clean Air Filter by soaking in petrol, squeeze out all dirt in the sponge filter and then leave to dry before re-installing.
Make sure that after cleaning the carb, you refit the float bowl correctly, the deepest part of the bowl is where the float opens and closes, DO NOT over tighten the bowl, if you do, you will distort it and then you will have problems.
Fit new spark plug.
Richard, August 2008