A friend of mine is having trouble with his mower. It's a Husqvarna with a 21hp Kohler Courage engine (SV610). A few weeks ago I changed the oil, filter, fuel filter, and plug in his mower for him. He's had the mower a few years and had never done anything to it. When I drained the oil very little drained out- maybe 1 cup. Started it and ran it for a bit after getting done with everything and it ran fine. He started it for the first time since today and it ran very poorly. Sputtering, blowing out black smoke sometimes, etc. Starts right up though. I pulled the carb off (Walbro) , cleaned it and blew it out with compressed air. Looked pretty good inside- a little bit of dirt in the bottom of the bowl. Put it back on and it runs the same as it did before. Not sure where to go from here. Thanks for the help.