was mowing the grass, the engine starting running rough, cut off, then would not crank, waiting till it cooled off, then cranked--still running poorly, very little power, backfiring and black smoke coming out exhaust. Changed the fuel filter, air filter, oil filter and spark plug. Now can not get the engine to crank. Did not drain the gas, but gas was bought the day before, should be fresh. What can I do to get it to crank. Thanks.