Sounds like an air leak to me too,and the most likely spot is the joint between the plastic back plate and the carb or cylinder.Be careful tightening those long coarse thread screws into the plastic as they will strip easily.If an air leak is not apparent then go back to basics,and check through all fasteners,and the entire fuel system.I agree any problem with the reed valve is unlikely and I would mess with it only if all else fails......good luck and let us know how you make out.Any gas experts in your club?sometimes a fresh pair of eyes will spot the obvious that we miss in our upset over the engine going sour...