challenger extrem petrol mower?

See the answer to question "Briggs and Stratton Motor" posted 41 questions after yours.
Also in your case, there is a very fine filter in the end of the plastic pick-up tube inside the fuel tank. To get at it you need to separate the carb from the tank although this tends to cause running problems not startup problems. Try it as a last resort

GB in Wales
February 2013