Lawn moyer repair.? Any ideas as to why my lawn mower 4 years mature will...

Any ideas as to why my lawn mower 4 years mature will start on the first pull and then only just slowly stop. I put in a new spark plug checked the oil-no luck .I never have a problem in the past other started the first pull. THANKS
Answers:    at four years it may have rust/dirt contained by the fuel tank restricting fuel to carb. which may be full of same. If you clean nouns filter and fuel system it run like its' old alien self.
sounds like you got build up of refuse in gas line- filter or carburetor-enough gas it getting through to crank but not enough to run