If this is the type with a float chamber on the carb, remove it and check that the main jet is clear. If you don't want to remove the carb, use carb cleaner and spray it through the jet from underneath.
If it's the 'tank under the carb' type, there is a rubber diaphragm between the carb and tank. Replace it. Cures 99% of fuel problems on these engines.
Put a new plug in it while you're at it.
Phil Saunders
November 2007