It could potentially be a bunch of different things. She may not be getting enough fuel in the carb as the float or needle valve could be stuck as well as the carb could be gummed up. When was the last time you changed the air filter or cleaned the oil/intake?
I would close the fuel petcock and shut hte fuel off, then pull the plug on the bottom of the carburetor to drain it. With it out, turn the fuel on and if the fuel is not looking like its "peeing" out of the carburetor its the fuel line. Once you determine how much fuel is coming out (trickling, barely running, flowing) turn the gas off. If it is flowing out, take some carb cleaner and spray it up through the hole, but don't have your face next to the hole as you don't want that stuff in your eyes. Let it sit, then blast it again. Put the screw/bolt back in and turn on the fuel and try to start it.
I would also pull the spark plugs and look at the tip, also see if its the correct gap and adjust accordingly if not replace them if they haven't been changed in years.. Clean the air filter and intake. Re-gap the points. And try to start her up.