Hi Andrew. The smoke you're seeing is unburnt oil. It could be the air filter is clogged or maybe the spark plug cap is dodgy. I take it the oil is pre-mixed with the fuel, have you mistakenly added too much oil (it's easily done). Also the quality of oil can make a huge difference, a safe bet would be to try a good quality synthetic oil.
If the air filter looks ok and plug cap isn't damaged I'd mix fresh fuel using synthetic oil and give that a go. Good luck with it.