I recently acquired a 1971 7hp Sears Ted Williams outboard. When I purchased the outboard it had set for 2 years and not been fired up. Since the purchase I have changed the spark plug and cleaned the carb. The engine will now fire up and run for 10 or 20 seconds very rough before it dies. Any ideas as to what is causing this and what I good solution may be to solve this problem? Any ideas will certainly be appreciated.