Small Engines (Lawn Mowers, etc.): Murray lawn mower, briggs and stratton, push mower

Question
I bought a new push mower a couple of months ago and all of a sudden it won't run right.  I changed the spark plug and put a new filter.  still sounds like i is missing and going to die and usually does and going through 3 times the gas.  do you have any ideas?  I thought it was bad gas. it is a briggs and stratton 4.5 hp
Thanks for your help  

Answer
you probably need a diaphram it sounds like it sucked part of it in and fuel out of the tank it costs about 3$ at a local parts store or if its still under warranty usually 3 mo you can take it to an authorized briggs dealer and have it repaired for free