I have a Cub Cadet series 2000 tractor, model 2185, with 18 hp Kohler engine. When trying to start it, the engine backfires and turns over slow. Verified getting gas and spark to the cylinders. Both cylinders have good compression. I found by removing the spark plug from the left cylinder, the engine cranks faster and will start, only running on one cylinder. Found yellow spark at the spark plugs, so I pulled the engine and replaced both ignition coils. After re-installing the engine, am getting the same results. Any ideas? Thanks.