toro self propelled problem

I have a Toro lawn mower that I am trying desperately hard to fix. It will crank but won't run. At first it would crank and run a couple of minutes. Now it only runs a couple of seconds. I'm a single chick and I can't afford to take it anywhere. I have tried everything I've been able to find online.
Here are are the things that I've tried. I've replaced the fuel and cleaned the carburetor. I've changed the oil, new air filter, new primer bulb, new spark plug and checked the fly wheel key. I have also sanded the rust from the contacts on the fly wheel.
If anyone else has a suggestion I'd appreciate you sharing it.

If it will only run a few seconds, it sounds like a fuel issue. When you took the carb bowl screw loose, did fuel run out? When cleaning the carb, did you make sure the needle was seated correctly when you put it back together? Is there something in the bottom of the gas tank that might be blocking fuel? Are the fuel lines clean? Hope some of this helps. If not, what make Toro do you have (i.e. model number and how old is it)?