Jim is right on with his black smoke/blue smoke explanation.
You need to see to this very soon if you want to avoid that overhaul. A rich condition means that raw fuel is washing the oil off of the cylinder walls. This will lead this engine to an early grave.
Mileage really is no indicator of overhaul time. Some engines need it very soon, while a properly cared for, carefully driven engine, especially an M103 can go hundreds of thousands of miles before the need. Overhaul time is determined strictly by condition.
See to that fuel system soon.
Have a great day,