I have an exmark lazer with a 23hp kawi. It has about 200 hours. Recently on start-ups I get a small cloud of black sooty smoke. Otherwise it runs strong. Oil level is correct. I assume this is from some sort of rich fuel mix situation? What do I need to look for in terms of problems? Could it be the choke not returning to the fully open position? Should I check the plugs? Your ideas appreciated.