My lawnmower will start but won't stay on for more than a few seconds, it has gas in it and I changed the oil. What are possible problems

What type of mower are you using?

I'm not sure what it would be exactly on yours, but you need to start it in "choke" mode, and after the engine gets going, change it to "run".

There may be another problem, but that is the first that comes to mind.

Hi,
Just a guess, but it sounds like a fuel delivery problem. Gas can get to the circuits, but not as fast or in the quantity required. Partially plugged main jet would be my guess.
Peace,
Clarke