I have a new Craftsman 24" snow thrower. Its 1 yr. old. I changed the oil, gas, and spark plug. When i get it running. It sounds like the engine is pulsing. Its not a cosistant run. I can see the trodle on the carburetor going back and forth. The impeller assembly is turning and sprial ass. is working. but the engine does not sound right? do i need to adjust the carburetor? if so. how? thanks, Hank. snow is comming soon.