I would question when the backfire is happening. Does the engine surge at all? Any smoke? I’ve had dozens of these engines, and I have never had to do any valve work on any of them (L-head versions; OHV is a different animal). A clean carburetor is a must. In rare instances, a sheared flywheel key can cause all sorts of backfiring headaches. Only had this happen a few times; usually from foreign object strike with lawnmower blade or snowblower auger.
