21 HP B&S blowing black smoke out of the muffler (YTS 3000)

[wgmarti]

Model 917.28922 on the manual cover or Model 917.289223 under the seat (YTS 3000). Blowing black smoke from the muffler when you start it up and when you put a load on the engine such as engaging the mower and at times when mowing the grass it throws out black smoke. Also has a smell but not sure what it is. When I took the plug out it is black with soot on it. Today I put a new air filter, oil and filter, fuel filter, spark plug. I also took the carb bowl off to see if it was gunked up but it looked good no varnish the gas in the bowl was clean the float was moving. After completing the above started it up no black smoke but after a few minutes back to black smoke. Even though it smokes the lawn tractor runs, it sputters every now and then but was able cut the grass and it idles although a little rough. The engine is a 21 HP, B&S Platinum with XRD engine family ABSXS.5402VB. The mower is three years old. Haven稚 worked on Lawn tractors but have rebuilt my rancher carb and triple mikuni's on my jet skis. Any suggestions are appreciated. Thanks.