Mountfield SP470 Mower fuel mix - too much petrol?

Hi,
Wonder if any one can help. I have a Moutfield SP470 that is running too rich. It works ok from cold, but then as warms up it starts to splutter and the engine doesnt run smooth at all. The spark plug is also becoming very sooty which suggests the fuel/air mix isn't right.

I have looked and there doesnt appear to be any way of adjusting the mix. Can anyone help?