How can I make my McCulloch Titanium 46SD mower run properly?

I have a McCulloch Titanium 46SD mower. The engine seriously "hunts" when going. I took out the spark plug and noticed it was completely black. I cleaned it but it has no effect, the engine still hunts. I think at the end of last year the mower was filled with a 2 stroke oil / petrol mix in error instead of standard unleaded petrol.As there is a very long waiting list at this time of the year for repairs, can anyone suggest if I could fix this.
Many thanks for any replies