Why won't my lawnmower start unless I pour gas on the air filter?

i got a lawn mower off the roadside (I'm 12) as a result it didn't work( go figure) I'm very mechanical so i tryed to fix her and i needed a way to make money. it's an old snapper mower, i think its a briggs and stratton engine.anyway i replaced the rusty spark plug and put some other things on, and for the life of me could not get it started. the only thing that worked was when i cleaned out the air filter, and drenched it with gas. and finally she had some smoke out of the muffler, but she started!then i tried again in like 20 minutes and she started then again in like 4 hours and nothing. should i get a new air filter? heelp!!! please