Hi, my Mountfield mower also does this. Mountfields told my Dad that it's the diaphragm gone on the carburettor. In the case of the Mountfield the carb is a sealed unit and I can apparently buy one for about £14 and fit it myself. Please don't take it as gospel that this is what's wrong with my mower but it may well be and is one avenue for you to investigate
Elaine
May 2008