I prime my Techumseh, it starts, runs for 1-2 seconds then cuts out. ?

Could you please help me?
My Performance Power 420 mower has a lovely Techumseh 4 stroke motor which is normally brilliantly reliable. Last week I primed it, it started but cut out after 1-2 seconds. It keeps doing this and won't run. What do you think it could be please? Does it need a "mower doctor" or just some fresh fuel?
I would value your opinion.
Many thanks
Tony