poor running mountfield

Can anyone advise?When i start my mountfield sp470, it starts and runs ok,but after about 10 seconds, it runs lumpy,and the spark plug goes sooty.l read on another "sick mower post" about a sticky choke gizmo.Although the mower has not been used much,it is over 2 years old,and i have no booklet with it.Any ideas ???

 

Well it certainly sounds as though the mixture is too rich so the sticky choke could well be the problem.