I forgot to ask... Are you turning the fuel off after shutting down? The fuel needle in the carburetor is not meant hold fuel back when not running. Only the fuel valve at the bottom of the tank does this job. Also, non-Ethanol fuel is best in these engines, if you can get it in your area.
I still have an '87 Snapper RER 30 inch cut with the original B&S Flat head 8HP. Those riders are some good, reliable basic cutting machines.