CHANIN SAW STARTS BUT IT SHUTTS OFF AFTER 2 OR 3 SECONDS?

I HAVE A POULAN WILD THING 18'' CHANIN SAW FOR MORE THAN FOUR YEARS. UNITL RECENTLY IT WAS DOING FINE BUT JUST SUDDENLY, IT STARTS SHUTTING DOWN AFTER I STARTS IT UP. I CHECKED THE SPONGE AIR FILTER, IT WAS COVERED WITH SAW DUST, SO I CLEANED, WASHED AND DRY IT UP AND PUT IT BACK AND START CRANKING IT. IT RUNS BUT BUT THE CHOKE WOULD JUST POP BACK IN AFTER 2-3 SECONDS.