Kohler CV23S One Cylinder Backfires

I own a Wright Sentar mower with a Kohler CV23S engine that has 840 run hours on it. Three weeks ago it was running fine, suddenly the engine RPM dropped, and it began to backfire, blowing black smoke out of the muffler. I installed a new gas filter and spark plugs. Both cylinders show a strong spark and seem to be getting gas but only the one by the fuel pump seems to have any power. The other cylinder backfires with every spark, blowing black smoke out of the muffler. I need help determining what the problem is. Has anyone had a similar problem, and how did you fix it?
Thanks.