Erratic idle

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Push mower. Wal-Mart source. 4.75hp B&S. 22" cut. 2 years old. Idles good when cold for about two minutes at 60 - 80 F ambient temperature. After that, engine idle cycles like this: normal idle speed for about 5 seconds > sputters/slows down/some black smoke for about 5 seconds, back to normal again, continuous cycle. If under load/mowing decent height grass, doesn't happen. Mower happily cuts the grass without protest.
How do I fix the warm idle problem? Dave
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