On Wed, 02 Jul 2008 13:21:54 -0500, Jeff Dege wrote:

I have a lawnmower that had been running fine. A week or so ago, it
started stalling out, just after I'd started running it.

It'd start easy, run for about 20 seconds, then sputter out. It did this
regardless of whether I was mowing anything or not. Load didn't seem to
have anything to do with it.

I was thinking "gas or oxygen", so I removed the air filter and it ran
fine. I then ordered a replacement air filter.

With the replacement air filter in place, I get the same behavior. It
starts easily, runs for a bit, then stalls. Without the air filter, it
runs fine.

Any ideas?

Most carb difficulties come from the main jet being obstructed either
partially or completely and/or the needle valve/seat or float preventing
the proper level of fuel being maintained in the bowl. Not knowing what
motor/carb you have makes it difficult to speculate on why it runs with
the filter off and not on. Not knowing if the engine stalls because it is
getting too rich of a mixture or too lean makes it even more difficult.
If you've never had a carb apart you might just want to take it to someone
who has and can clean it, replace the needle and seat and gaskets.