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