Cub Cadet Model 2145 Mower about turned over and had to turn down hill quick. Engine shut off and when I crank it back up, engine was backfiring thru carburetor and not running smooth. When I gave it more gas the engine shut off. I continued to try to get engine to run by cranking it, but didn't run right. I changed the spark plug but that didn't help much. Left the plug out, and turned the engine over a few times. Took filter off and check for signs of oil. All appeared to be clean. When I cranked the engine with the filter off it seems to run smoother. When I put filter back on and gave more gas, it begin to backfire again. Currently not running smooth at all. Please help!!!