I would shut the fuel valve and remove the bowl of the carburetor and inspect for water or debris. While the bowl is off, check the flow of fuel by watching the fuel trickle out into a pan. If the fuel cap is venting poorly, you should be able to compare the flow with the cap tight vs loose to know.

Thanks for your reply. I tried that and I did not see any water or debris and the cap seems to be venting properly. Also, I noticed that when I first crank it up when the engine is cold, it will run perfect for about 1 minute and then the previously described behavior begins. I'm not sure if this changes the scenario or not. Do you have any other suggestions?

This post was edited by rikstick on Sun, Jul 13, 14 at 11:47

With that title, I had to read it. Can you imagine what people think when they see him mowing, tilting, mowing...? Unfortunately I don't have a real answer for it. Mowers will typically run differently when tilted to one side or the other because it affects carb float level. Maybe you have a sticking float. Pick up the carb side and drop it from about 6" to see if that jars it loose. You should be using a fuel additive if you're using Ethanol-blended gas. Some stabilizers are made for Ethanol fuels and also help clean the fuel system.

I replaced the carburetor (I bought one from a seller on Ebay named Just click-n-save for $39 and she runs like a charm.