Does your tractor have the little electrical solonoid on the bottom of the carb fuel bowl? Those can get gummy with this new gas and get sluggish or not open all the way. They shut off gas to the carb jets when you turn off the ignition. Kohler factory reps actualy tell dealerships to have customers shut down thier engines at full throttle to help stop the backfire. Goes against everything I have learned as a mechanic. I personaly throttle down to mid rpm just prior to turning off the mower deck, then hold that rpm till I get to the garage, then drop to idle as I back in, then shut it down. Rarely get a backfire. Seems to work with both new and old engines. The backfire is gas that makes it thru the carb after you kill the ignition, collects in the hot muffler and if there is enough there, "BANG!" I have seen ends of mufflers blown out by this, and they are not cheap. Having to use choke every time, even just for a split second is normal for newer engines. Same for the backfire under certain shut down conditions. Mike