They will be fouled with oil or with fuel. Causes of fuel fouling are: jets too big, choke stuck open, fuel level too high, sticking float, or debris in the needle jet.
Oil fouling can be caused by bad rings or valve guides. A dirty air filter could make oil or gas fouling worse.
Fouled spark plugs are not that common on four-strokes. You didn't think it was a 2-stroke and mix oil with the fuel, did you? A weak spark or too cold a plug could also contribute, but the underlying cause will probably still be due to rings or guides in poor condition.
