i have a 1 kva portable genny i can get it to run but not for long about 30 to 40 mins then the plug gets very sooty and black and it cuts out untill i remove the plug and clean it and i am fed up with cleaning it
From what you have said it sounds as if the engine is running too rich
Might be caused by
Incorrect spark plug - Electrode too hard
air / fuel mixture needs adjusting
Blocked air filter
If 2 stroke engine petrol/oil mix incorrect - too much oil - check manufacturers spec