My john deere 345 mower is running rough ,popping, no power, backfiring, using over halve the gas it should, with some black smoke. can mow halve may yard, take a brake and it will start right up, but will not stay running. have replaced the plugs, shot off fuel solenoid, fuel filter, and the carb and air filter are clean. never posted in a forum before, so thank you in advance.