Small Engines (Lawn Mowers, etc.): lawnmower engine, hp briggs, adjustment screws

Question
I have a 6.5 hp briggs and stratten engine on our 3 year old self propelled lawnmower.  The mower will start and run rough for awhile and then begin missing and loosing power.  The spark plug is removed and it becomes black. After cleaning the plug the mower will run better for awile but far from perfect. I have changed the air cleaner and replaced the spark plug.  What might be causing this problem? Coil or carburator?

Answer
Hi Howard,

It sounds like the carb is running rich.  Some carbs have mixture adjustment screws.  Try turning the larger screw clockwise 1/8 to 1/4 turn at a time.  
Coils usually fail totally, rather than get weak.

Let me know how you make out.
Michael