Hi Treacle
Don't know how used you are to setting up engines, but maybe you are running it too rich - although it could be a number of other things, too. Easiest way is to get someone experienced at the flying field to show you how to 'twiddle' the top and bottom end knobs/screws. If that doesn't solve it, then whoever's helping ought to be able to see what else might be wrong.

ps If it is a new engine, you did follow the maker's instructions about running it in, didn't you?

Good luck.