how to fix a sovereign lawn mower /brigg and stratton 3.5 engine?

how to fix my sovereign lawn mower fitted with a briggs and stratton engine xc35 classic . the problem is that i cant get the engine to run more than a few moments i get it to run when i prime it but it soon stops i have cleand out the tank the pipes all are clean so what do i do now . the mower was good all last year and was in good order when i put it away for the winter . now it just wont go ANDY