The backfiring upon shut down is normal for those. You can fix it with a carburetor re-jet. Since it's a simple, air cooled single-carb engine, it should not cost much at all to re-jet it. I have heard that it likes a #152 main, a #50 pilot and 1/4 turn out (from the stock setting) on the idle mixture screw. With these changes, it should make more and smoother power too. It will use a bit more gas though.
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