I get allot of used mowers, actually they are mowers that people want to throw away. I take them and go to one of the local briggs and stratton reps. and get the same things every time... A $7.00 carburetor rebuild kit and a $3.00 spark plug. It works every time. My suggestion is look around to see where your local distributor is... I was extremely surprised at how many are around once I started looking. I have not purchased a brand new mower for around 10yrs. The first throw out mower I got (an older self propelled) is still going strong (this is after someone put it out for the garbage man... I know, I actually went dumpster diving). You could definitely do what the others suggest and replace the diaphram (that is probably your problem) but at the cost of a full carb rebuild kit... it just runs better all around...