1973 IS rear bath. Too often unpleasant odor comes
out of toilet when flushing. Have tested with clean water. Bowl 1/2 full, push pedal to open valve ~ 1/2 way. Before bowl is MT air ( unpleasant air ) burps up through the remaining water. Could the tank vent line be plugged? I would think water entering the tank would force the air up and out of the vent pipe ???