My Murray 5.0 push lawnmower won't start. It briefly started and when I started to mow, it cut off. This happened a couple of months ago so I checked the air filter and basically knocked all the dirt out and it started like it was brand new. Now it's won't start again. I knocked the dirt out of the air filter, added oil, and changed the spark plug. No luck. When I try to crank the engine, it barely starts but dies seconds later. I changed the oil at the beginning of the summer so I didn't think it was necessary to change again so soon. Help! :confused:

The oil's not the problem... if the air filter was so dirty, replace it!.
It sounds as though you got trash in the carb... which would mean taking it off, tearing it apart and at the most blowing it out with carb cleaner through any and all holes.

I'll try both and let you know what happened. Thanks.