2000 yreg nisan micra wont start?

when turning engine over it has a spark but it then seems to disappear after a couple of seconds, and just wont fire up tried putting fuel down carb but it just pops back through carb, some times it will start and run great for a day or so then wont start again.
have also replaced both sensors cam and crank and ignition switch, and it still wont start, is there anyone who knows about these nissans.