You've installed the new spark plug.
You've installed a new air filter.
You've cleaned the jet on the nut which holds on the carb bowl.
You've tightened the two bolts VERY GENTLY which join the carb with the manifold.
You've made sure the gas tank is clean and has fresh gas.
You've changed the oil and put in the proper SAE 30W oil (20 oz.)....
AND THE MOWER STILL STARTS AND THEN DIES!!???
DO NOT MELT DOWN JUST YET!
I had this problem and I think I found the formula for the long term fix. If you have the skills and tools to take off the red cover AND the black metal cover which houses the start cord mechanism - here's what you do:
First, take a small wire brush (a 1/2" copper fitting brush works very well)and clean off the magneto surfaces on both the coil and the flywheel. If you don't know where this is, just follow the spark plug wire from the spark plug up to the coil and there it is. There is one surface on the flywheel and three surfaces on the coil.
Secondly, there is a very short black hose leading from the carb to the engine which you cannot get to without removing the black metal cover. This hose comes from the factory without clamps, and from what I could figure out, with age this hose loses vacuum when the engine gets warm and the hose expands slightly. That's why the mower will run fine for a few minutes and then all of a sudden after the hose warms up a little, expands and loses vacuum, the mower does the start and die routine which we all know can drive you nuts.
Get two spring-type hose clamps which are identical to the ones attached from the gas tank to the carb. These are available at Ace hardware for around 40 cents apiece and they are 1/2" O.D. size. PUT THESE CLAMPS ON THE SHORT BLACK HOSE. This is the "secret" ingredient to the long term fix.
Put the covers back on. Don't overtighten the bolts. And get back to work!