Small Engines (Lawn Mowers, etc.): Carburetor problem, briggs stratton, hp engine

Question
I have a DR Chipper with a Briggs & Stratton model 422437 18 hp engine. I bought it new four years ago. Last year when I was chipping, the engine started to sound like it was straining and that is when I noticed the muffler had gotten so hot that it was melting the plastic belt guard. I've taken the carburetor apart, cleaned it physically and with carb cleaner. The only way I can keep the engine running is with the choke pretty much closed so something is leaning it out evidently. There is good fuel flow to the carb and the diaphragm inside it looks good. Any ideas or suggestion for further trouble-shooting? Thank you for your time.  

Answer
Get it running as you say you can and spray carb cleaner around the gaskets where it mounts to the engine and around any throttle or butterfly stems to check if its sucking air from some other area. It will change how it runs when you do this if it is. The gasket can be replaced if its a stem replace the carb. If it is not sucking air from these areas then its the the carburetor not allowing enough gas to pass through which is why you have to choke off the air flow to keep it running. A rebuild kit may do the trick. Most of the time we just replace these carburetors as oposed to rebuilding them.