Small Engines (Lawn Mowers, etc.): john deere stx38, acre yard, dense smoke

Question
I just bought a used stx38 and have a very flat, 3/4 acre yard. the machine is extremely clean, starts up easily and runs nicely...until i get about 75% done mowing the lawn, when it emits a large amount of white, dense smoke from the exhaust pipe. the oil is fine, clean and full. i am really not happy about this, as the 30 day parts warranty is about to expire and the company (a deere dealership) doesn't know what is wrong. the smoke only happens after the mower has been running for a while. any help would be greatly appreciated. one of my friends said that it may be a carburetor problem letting the mix get too rich. I have no idea as i have no experience with engine work.

Answer
Hello Sam:

Explain in More Detail the White Smoke. Does it Puff the Smoke and the Smoke Quits or Does it Continue to Smoke? How Long is the Engine Operating (Cutting Grass) When this Happens? Thanks.

Respectfully

John

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Respectfully

John