I'm running a snapper 21 with the wisconsin robin 5hp engine. Sometimes when I stop the mower to empty the bag, it will backfire when I restart the mower. It only backfires once, on the first pull, after i've re-started it. The mower has been maintained impeccably, and runs really well. No smoke, runs smoothly, plenty of power, etc... Anyone know why I get that backfire after a quick break to empty bag?