Problem: My lawnmower began quitting after running for 3-5 minutes. I changed the oil, fuel filter & air filter. Started the lawnmower up & it ran for 2-3 hrs. I put the mower away for the evening & came back the next morning to finish cutting the grass. Mower ran for about 30 minutes & then started doing the same 3-5 minutes & quitting.

Any ideas?

Hi,
Based on your description, I would suspect the quality/cleanliness of the fuel and think that it is clogging the fuel filter. I might also suspect the tank/cap vent a bit.
Peace,
Clarke