Tecumseh Horizontal Tiller Engine 8 Hp, 1" X 2 57/64" Shaft, Modelno. Hm80 - 155656x "coleman generator wheel kit" Questions

...coleman generator. won"t stabilize, backfires. new plug, fresh gas 'carb cleaned. starts first time'new air filter. any suggestions? check to see if it has a smeared or worn key way it is what times ...