Hello Mr h'
I recently had this problem and solved it by altering the mixture, my neighbour heard the racket (he was a mechanic) and said it was running rich. On my carburettor the mixture screw was the one on the right and was operated by using an allen key. When I turned the screw on the left that increased the speed of the engine, yours might be the other way around. Hope this helps.
Alan, September 2008
