I have a mower with a Briggs and Stratton Q 4.5 engine that's given good service over the past 5 years. Recently, it started surging / lagging after about 5 minutes or so of use; it starts up fine and runs well initially. I replaced the carb spring, cleaned the air cleaner, cleaned out the gas tank and filter. When I removed the spark plug, it had thick black deposits. I replaced the plug, but after running for only about 20 minutes, it also had lots of thick black soot on it. It's behaving like it's not getting enough air / running too rich, but I don't know what else to do. Suggestions? Thanks.