sprint 375 model garden marco pole 45 petrol lawnmower

my lawnmower starts after i prime it 3 times as stated in the chassis, but after 5 minutes it starts ti lose revs and then cuts out. i have cleaned the holes in the petrol cap. any ideas of what the problem is ,someone suggested that it could be the diaphram.many thanks brendan

Brendan Murphy, April 2011