my Briggs and Stratton lawn mower engine was running rough. I checked the oil and it was very low, so i added oil. i was concerned b/c I check the oil level . that helped for a minute then was running rough, then not at all. i opened the air filter on the carburetor and a lot of black gunk in the carb, had the consistency of oily dirt or sand. I scooped out a bout a tbsp work and tried to clean it out with a rag, put a few drops of gas in- still will not start. Does this mean the engine is dead/ should i buy a new one- don't want to spend $75 for the shop to tell me that.