riello burner problem?

I have a riello balanced flue burner fitted to a boulter fire bird boiler,the burner ignite but after a short time the flame goes out the motor does not stop and the burner relights, this process is constantly repeated. If I remove the air input tube from the burner the burner runs continuously as it should do. The tube is not blocked,kinked or the flue restricted. Any ideas.

graham
January 2014