I recently purchased a freshly serviced used John Deere LT150 and mowed the lawn for the first time today cutting roughly 1/2 acre. It starts beautiful and runs smooth, my question is that during the process I turned off the engine a few times and each time the engine (15hp kohler) had a single loud knock and some afterrun or backfire. It would start right back up when I was ready to go again but I'm concerned this is not normal...what should I be looking for?