when i try and start it it floods what causes this?

my car ran great until recently. my dad just replaced the spark plugs becuase we thuoght this caused the flooding but it didnt help. my car will start and then sometimes die or i can put my foot all the way down on the gas peddle and it will eventually start up but it seems like its running at higher rpms. im not to sure of that though it just sounds louder when started it drives fine.