Put some seafoam in the fuel tank point it toward the big part of the lake and run it hard for long periods of time. You want to keep an eye on the engine for exhaust leaks as a possible cause of the black oil from up top I just use WD on the powerhead and spray off with water as the engine is running and try and keep it away from the wiring as much as possible. Or you can spray dish soap all over the powerhead and look for bubbles while running if it stumbles or stalls you probably have a vacuum leak somewhere. Once you get it good and clean it makes diagnostic easier. So what was the problem with it not stopping when you turned the key off?