engine keeps sputtering and backfiring when giving gas or under load
I have a briggs and Stratton 23 ohv engine on a john deere zero turn 425 mower. It ran perfectly last week. went to start it this week to mow and now it sputters, black smoke and backfires all when trying to give it gas. it will not run fast nor can I engage the blades as the load makes it do the same thing and cut off. I changed the plugs, cleaned the carb, checked the valves, the blades move freely. It starts up and will idle and run on the lowest throttle setting but it will not do anything else. As soon as I try to push the throttle to fast and engage the blades it goes crazy. I don't know what else to try.