throttle on craftsman lawnmower vascilates

I have a craftsman 21" lawn mower. (model 917.388510) Every year, I do minimal maintenance including draining the gas and oil, replace the spark-plug, cleaning the air filter and occasionally replacing the blade.
This year, I've noticed that while it starts easily and stays running, the speed increases then decreases continuously. The throttle linkage is clean, clear and moves easily. I see no set-screw controls to govern the speed. If I pull and hold the throttle, it runs consistently but I can't get it to remain that way, even after tightening the spring pulling the throttle.
Any idea what it might be? Thanks
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Happened to me about 2 years ago.
There is a retaining spring on the carburetor linkage. Not sure if that's the spring you are talking about.
If it's not, check that it is still there and attached.
--
Dan Espen

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Happened to me twice (different tools). One was bad gas in a snow blower. Once I blew through that it was fine. The other was a plugged air filter in a mower. Blew it out and all was fine.
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On May 12, 8:10 pm, " snipped-for-privacy@att.bizzzzzzzzzzzz"

I'd put my money on the clogged air filter.
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wrote:

In addition to the other things people have suggested, I have had water in the gas cause a mower to act this way.
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