I sorted this by adjusting the pilot screw on the front underneath of the carb. Its awkward to get at but with the engine running turn it either way 'till it sounds good. The spec says 1.5 turns from closed, but mine is about 2.5 turns. After doing this I have had no problems since. Course I did get the carb off and service it first.
All the best , Steve
Stephen Scullion, January 2008