My Lawnmower empress 16?

hi I have a empress 16 mountfield and although it starts it chugs and if I put it on the grass cuts out. I read that it sounds like fuel to rich as I looked at the spark plug and it was sooty. I am unsure if I am turning the right screw and I also have put some Redex in the air filter box and petrol I thought it was starting up put it soon stopped again can you giude me to the correct screw for fuel mixer and if there is anything else I can do I have no engine experience do I have to take it to a professional for a service or is there something I can do to get it working thank you