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

On May 12, 5:02*pm, sammy wrote:
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

Some mowers have an "air govenor" to maintain a constant speed while
you are mowing. It's just a flap in the cooling air stream. If the
fan slows, it opens the thottle slightly to boost the speed.
Your problem may be related to this device. Can stick/get grass caught
up in it.

sammy writes:

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?

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

On Thu, 12 May 2011 09:02:05 -0700 (PDT), sammy wrote:

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?

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.

On May 12, 8:10*pm, "
wrote:
On Thu, 12 May 2011 09:02:05 -0700 (PDT), sammy wrote:
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?

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.- Hide quoted text -

- Show quoted text -

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

On Thu, 12 May 2011 09:02:05 -0700 (PDT), sammy
wrote:

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


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