Mount field hp414 starting problem?

When I prime my mount field lawn mower which has a rs100 motor, I press the prime button a couple of times, the lawn mower starts briefly then cuts out. This happens every time. I have replaced the air filter, and the gasket seals.
Someone mentioned the diaphragm might need replacing but I'm not sure where this is?
It is a mount field hp414.

Andy w 4303
April 2015