I have a Snapper 21" Comm. self-prop. with the above engine. I soaked the carb in carburetor cleaner, put in new needle, float and gaskets. Fresh fuel. Good oil level. Good gravity flow from the fuel tank. Starts great, idles great, mows great for about 50' then loses power. Almost dies, runs rough. If I try to keep mowing, it dies immediately. No longer idles smoothly. Then its hard to start. If I let it cool a few hours, it starts up great again. Any ideas what might cause this? Thanks in advance.