One other check you might make before taking the carburetor apart: take off the air cleaner and start it (using the fuel-in-the-plughole method if necessary) to find out if its puffing air and fuel back through the carburetor. If it is,remove the muffler and see if that cures everything. Once in a while some insects build a nest in there and block everything up. It is also possible for a huge clump of carbon to build up, then shift so it's suddenly in the way of the exhaust stream. Blocked exhaust is a fair fit for your symptoms.
