Yard mechine lawn mower (push)?

so i got this mower the other day from the flea market i used the gas which was already in it, then went to use my gas well the mower started to putt which it wasnt doing it with the gas in it before so i figure i got old gas so i ran tht out and then put fresh in it now its still putting i put a new spark plug on it and i cant figure it out i need help what could be the problem???????????